The "Undefined symbols for architecture..._mexFunction" error is usually caused by a missing mexFunction function.
In C/C++ applications, the program entry point is usually defined using the main function which has a specific signature:
int main() {...}
int main( int argc, char *argv[] ) {...}
Similarly, the entry point in a MATLAB MEX file is defined using the mexFunction:
void mexFunction( int nlhs, mxArray *plhs[], int prhs, const mxArray *prhs[] ) {...}
https://www.mathworks.com/help/matlab/apiref/mexfunction.html
If you add a mexFunction to your 'fftw_example.c', it can be compiled with the mex command. See some of the mex examples for more information:
https://www.mathworks.com/help/matlab/matlab_external/standalone-example.html
