[linux] fix RTGL1 determine win32/macosx/linux

https://github.com/sultim-t/RayTracedGL1

cmake --build .
[  5%] Built target rdatawad
[  6%] Built target prboomwad
[  6%] Building C object src/CMakeFiles/prboom-plus.dir/am_map.c.o
[  7%] Building C object src/CMakeFiles/prboom-plus.dir/doomdef.c.o
[  8%] Building C object src/CMakeFiles/prboom-plus.dir/doomstat.c.o
[  8%] Building C object src/CMakeFiles/prboom-plus.dir/dstrings.c.o
[  9%] Building C object src/CMakeFiles/prboom-plus.dir/d_deh.c.o
[ 10%] Building C object src/CMakeFiles/prboom-plus.dir/d_items.c.o
[ 10%] Building C object src/CMakeFiles/prboom-plus.dir/d_main.c.o
In file included from /home/build/prboom-RTX/prboom-plus-rt-rt-1.0/prboom2/src/d_main.c:73:
/home/build/prboom-RTX/prboom-plus-rt-rt-1.0/prboom2/src/i_video.h:65:5: warning: "RT_SEPARATE_HUD_SCALE" is not defined, evaluates to 0 [-Wundef]
   65 | #if RT_SEPARATE_HUD_SCALE
      |     ^~~~~~~~~~~~~~~~~~~~~
In file included from /home/build/prboom-RTX/prboom-plus-rt-rt-1.0/prboom2/src/RT/rt_common.h:29,
                 from /home/build/prboom-RTX/prboom-plus-rt-rt-1.0/prboom2/src/RT/rt_main.h:29,
                 from /home/build/prboom-RTX/prboom-plus-rt-rt-1.0/prboom2/src/d_main.c:101:
/home/build/prboom-RTX/RayTracedGL1-SharedLibrary/Include/RTGL1/RTGL1.h:41:14: fatal error: windows.h: No such file or directory
   41 |     #include <windows.h>
      |              ^~~~~~~~~~~
compilation terminated.
make[2]: *** [src/CMakeFiles/prboom-plus.dir/build.make:160: src/CMakeFiles/prboom-plus.dir/d_main.c.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:187: src/CMakeFiles/prboom-plus.dir/all] Error 2
make: *** [Makefile:136: all] Error

https://github.com/sultim-t/prboom-plus-rt/issues/1

sultim-t / prboom-plus-rt

[linux] fix RTGL1 determine win32/macosx/linux #2