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

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