clang: error: linker command failed with exit code 1 while compiling mex in MATLAB2015b on mac os X 10.13.4

Hi. I'm trying to compile a mex file on MAC OS X 10.13.4 using MATLAB 2015b and Xcode 9.3. When executing the command

mex -v volunwarpgradMEX.cpp

I get the following output:

    Verbose mode is on.
    Neither -compatibleArrayDims nor -largeArrayDims is selected.
       Using -compatibleArrayDims. In the future, MATLAB will require the use of
       -largeArrayDims and remove the -compatibleArrayDims option.
       For more information:
       http://www.mathworks.com/help/matlab/matlab_external/upgrading-mex-files-to-use-64-bit-api.html.
    ... Looking for compiler 'Xcode Clang++' ...
    ... Looking for environment variable 'DEVELOPER_DIR' ...No.
    ... Executing command 'xcode-select -print-path' ...Yes ('/Applications/Xcode.app/Contents/Developer').
    ... Looking for folder '/Applications/Xcode.app/Contents/Developer' ...Yes.
    ... Executing command 'which xcrun' ...Yes ('/usr/bin/xcrun').
    ... Looking for folder '/usr/bin' ...Yes.
    ... Executing command 'xcode-select -print-path' ...Yes ('/Applications/Xcode.app/Contents/Developer').
    ... Looking for folder '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk' ...No.
    ... Looking for folder '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk' ...No.
    ... Looking for folder '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk' ...Yes.
    ... Executing command 'xcode-select -print-path' ...Yes ('/Applications/Xcode.app/Contents/Developer').
    ... Looking for folder '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk' ...No.
    ... Looking for folder '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk' ...No.
    ... Looking for folder '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk' ...Yes.
    ... Executing command 'echo /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk | rev | cut -c1-10 | rev | egrep -oh '[0-9]+\.[0-9]+'' ...Yes ('10.13').
    Found installed compiler 'Xcode Clang++'.
    Options file details
    -------------------------------------------------------------------
      Compiler location: /Applications/Xcode.app/Contents/Developer
      Options file: /Users/donatas/.matlab/R2015b/mex_C++_maci64.xml
      CMDLINE100 : /usr/bin/xcrun -sdk macosx10.13 clang++ -c -DMX_COMPAT_32   -DMATLAB_MEX_FILE  -I"/Applications/MATLAB_R2015b.app/extern/include" -I"/Applications/MATLAB_R2015b.app/simulink/include" -fno-common -arch x86_64 -mmacosx-version-min=10.13 -fexceptions -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -fobjc-arc -std=c++11 -stdlib=libc++ -O2 -DNDEBUG /Users/donatas/Documents/SD_scratch/volunwarpgradMEX.cpp -o /var/folders/xs/xx9d6f1x4d7b1hs44z9gs45w0000gn/T//mex_15031778690944_17057/volunwarpgradMEX.o
      CMDLINE200 : /usr/bin/xcrun -sdk macosx10.13 clang++ -Wl,-twolevel_namespace -undefined error -arch x86_64 -mmacosx-version-min=10.13 -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -framework Cocoa -bundle  -Wl,-exported_symbols_list,"/Applications/MATLAB_R2015b.app/extern/lib/maci64/mexFunction.map" -stdlib=libc++ -O -Wl,-exported_symbols_list,"/Applications/MATLAB_R2015b.app/extern/lib/maci64/mexFunction.map" /var/folders/xs/xx9d6f1x4d7b1hs44z9gs45w0000gn/T//mex_15031778690944_17057/volunwarpgradMEX.o   -L"/Applications/MATLAB_R2015b.app/bin/maci64" -lmx -lmex -lmat -o volunwarpgradMEX.mexmaci64
      CMDLINE300 : rm -f /var/folders/xs/xx9d6f1x4d7b1hs44z9gs45w0000gn/T//mex_15031778690944_17057/volunwarpgradMEX.o
      CXX : /usr/bin/xcrun -sdk macosx10.13 clang++
      DEFINES : -DMX_COMPAT_32   -DMATLAB_MEX_FILE 
      MATLABMEX : -DMATLAB_MEX_FILE 
      CXXFLAGS : -fno-common -arch x86_64 -mmacosx-version-min=10.13 -fexceptions -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -fobjc-arc -std=c++11 -stdlib=libc++
      INCLUDE : -I"/Applications/MATLAB_R2015b.app/extern/include" -I"/Applications/MATLAB_R2015b.app/simulink/include"
      CXXOPTIMFLAGS : -O2 -DNDEBUG
      CXXDEBUGFLAGS : -g
      LDXX : /usr/bin/xcrun -sdk macosx10.13 clang++
      LDFLAGS : -Wl,-twolevel_namespace -undefined error -arch x86_64 -mmacosx-version-min=10.13 -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -framework Cocoa -bundle  -Wl,-exported_symbols_list,"/Applications/MATLAB_R2015b.app/extern/lib/maci64/mexFunction.map" -stdlib=libc++
      LDBUNDLE : -bundle 
      LINKEXPORT : -Wl,-exported_symbols_list,"/Applications/MATLAB_R2015b.app/extern/lib/maci64/mexFunction.map"
      LINKLIBS : -L"/Applications/MATLAB_R2015b.app/bin/maci64" -lmx -lmex -lmat
      LDOPTIMFLAGS : -O
      LDDEBUGFLAGS : -g
      OBJEXT : .o
      LDEXT : .mexmaci64
      DEVELOPER_DIR_CHECK : 
      XCODE_DIR : /Applications/Xcode.app/Contents/Developer
      XCRUN_DIR : /usr/bin
      ISYSROOT : /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk
      SDKVER : 10.13
      MATLABROOT : /Applications/MATLAB_R2015b.app
      ARCH : maci64
      SRC : /Users/donatas/Documents/SD_scratch/volunwarpgradMEX.cpp
      OBJ : /var/folders/xs/xx9d6f1x4d7b1hs44z9gs45w0000gn/T//mex_15031778690944_17057/volunwarpgradMEX.o
      OBJS : /var/folders/xs/xx9d6f1x4d7b1hs44z9gs45w0000gn/T//mex_15031778690944_17057/volunwarpgradMEX.o 
      SRCROOT : /Users/donatas/Documents/SD_scratch/volunwarpgradMEX
      DEF : /var/folders/xs/xx9d6f1x4d7b1hs44z9gs45w0000gn/T//mex_15031778690944_17057/volunwarpgradMEX.def
      EXP : volunwarpgradMEX.exp
      LIB : volunwarpgradMEX.lib
      EXE : volunwarpgradMEX.mexmaci64
      ILK : volunwarpgradMEX.ilk
      MANIFEST : volunwarpgradMEX.mexmaci64.manifest
      TEMPNAME : volunwarpgradMEX
      EXEDIR : 
      EXENAME : volunwarpgradMEX
      OPTIM : -O2 -DNDEBUG
      LINKOPTIM : -O
    -------------------------------------------------------------------
    Building with 'Xcode Clang++'.
    /usr/bin/xcrun -sdk macosx10.13 clang++ -c -DMX_COMPAT_32   -DMATLAB_MEX_FILE  -I"/Applications/MATLAB_R2015b.app/extern/include" -I"/Applications/MATLAB_R2015b.app/simulink/include" -fno-common -arch x86_64 -mmacosx-version-min=10.13 -fexceptions -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -fobjc-arc -std=c++11 -stdlib=libc++ -O2 -DNDEBUG /Users/donatas/Documents/SD_scratch/volunwarpgradMEX.cpp -o /var/folders/xs/xx9d6f1x4d7b1hs44z9gs45w0000gn/T//mex_15031778690944_17057/volunwarpgradMEX.o
    /usr/bin/xcrun -sdk macosx10.13 clang++ -Wl,-twolevel_namespace -undefined error -arch x86_64 -mmacosx-version-min=10.13 -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -framework Cocoa -bundle  -Wl,-exported_symbols_list,"/Applications/MATLAB_R2015b.app/extern/lib/maci64/mexFunction.map" -stdlib=libc++ -O -Wl,-exported_symbols_list,"/Applications/MATLAB_R2015b.app/extern/lib/maci64/mexFunction.map" /var/folders/xs/xx9d6f1x4d7b1hs44z9gs45w0000gn/T//mex_15031778690944_17057/volunwarpgradMEX.o   -L"/Applications/MATLAB_R2015b.app/bin/maci64" -lmx -lmex -lmat -o volunwarpgradMEX.mexmaci64
    Error using mex
    Undefined symbols for architecture x86_64:
      "index(int const*, int, int, int)", referenced from:
          _mexFunction in volunwarpgradMEX.o
      "getvxlval(double const*, int const*, double const*, double const*, int, int, double&)", referenced
      from:
          getuwlph(double const*, int const*, double const*, double const*, double const*, double const*,
          double*) in volunwarpgradMEX.o
          getJac(double const*, int, int const*, double const*, double const*, double const*, double const*,
          double, double const*, double const*, double const*) in volunwarpgradMEX.o
          _mexFunction in volunwarpgradMEX.o
    ld: symbol(s) not found for architecture x86_64
    clang: error: linker command failed with exit code 1 (use -v to see invocation)

Any ideas what could have gone wrong? I have files getvxlval.cpp and getvxlval.h in addition to volunwarpgradMEX.cpp that are linked together. The mex file are supposed to be error-free since it was compiled before on a linux platform.

Thank you for any suggestions!