Fix linker error for Windows mingw cross-compilation

[pclov3r]

Currently, if you attempt to try and cross compile liquid-dsp for Windows from Linux using mingw-w64 it will fail with

/usr/bin/x86_64-w64-mingw32-ld: cannot find -lc: No such file or directory
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/liquid.dir/build.make:404: libliquid.dll] Error 1
make[1]: *** [CMakeFiles/Makefile2:1056: CMakeFiles/liquid.dir/all] Error 2
make: *** [Makefile:156: all] Error 2

It appears some cleanup work has been done to make it much easier to cross compile but this remains an issue still.

This resolves the issue as MinGW provides it's own C runtime and this makes linking towards it conditional.

[pclov3r]

Would be awesome if there has been any updates on this and #403

Having to edit to compile it with mingw is slightly annoying.