To compile the xMat as a shared C library, use e.g.

gfortran -ffree-form -cpp -shared -fPIC -DCINTER xmat.f -o xmatc.so
rm *.mod

for Linux with GCC or from Windows with Intel compiler something similar to

ifort /free /fpp /dll /recursive /MT /DCINTER xmat.f /exe:xmatc.dll
del *.mod

Please note, that for compilation the macro CINTER has to be defined next to the flags for free format, preprocessing and dynamic linking (as shown).