Fix compile error on windows using mingw

[bredelings]

Hi,

I noted that on windows I get the following compile error:

"D:\a\_temp\msys64\mingw64\bin/ccache.EXE" "g++" "-Isubprojects/RE-flex-6.3.0/libreflex_static_lib.a.p" "-Isubprojects/RE-flex-6.3.0" "-I../subprojects/RE-flex-6.3.0" "-I../subprojects/RE-flex-6.3.0/include" "-fdiagnostics-color=always" "-DNDEBUG" "-D_FILE_OFFSET_BITS=64" "-Wall" "-Winvalid-pch" "-Wextra" "-Wpedantic" "-std=c++11" "-O3" "-g" -MD -MQ subprojects/RE-flex-6.3.0/libreflex_static_lib.a.p/lib_input.cpp.obj -MF "subprojects/RE-flex-6.3.0/libreflex_static_lib.a.p/lib_input.cpp.obj.d" -o subprojects/RE-flex-6.3.0/libreflex_static_lib.a.p/lib_input.cpp.obj "-c" ../subprojects/RE-flex-6.3.0/lib/input.cpp
../subprojects/RE-flex-6.3.0/lib/input.cpp:53:12: fatal error: sys/select.h: No such file or directory
   53 | #  include <sys/select.h>
      |            ^~~~~~~~~~~~~~
compilation terminated.

This is because I'm using the mingw compilers, and for some reason they are not treated as windows compilers. The current PR fixes the issue by treating MINGW just like other windows compilers, which seems like the correct fix.