Sets construction variables for MinGW (Minimal Gnu on Windows).

CC SHCCFLAGS CXX SHCXXFLAGS SHLINKFLAGS SHLINKCOM LDMODULECOM AS WINDOWSDEFPREFIX WINDOWSDEFSUFFIX SHOBJSUFFIX RC RCFLAGS RCINCFLAGS RCINCPREFIX RCINCSUFFIX RCCOM OBJSUFFIX LIBPREFIX LIBSUFFIX SHLINKCOMSTR RCCOMSTR