Determining if the __GLIBC__ exist failed with the following output:
Change Dir: /home/tux/Schreibtisch/Git_SDL_Script/SDL/SDL3_ttf/build_win64/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/gmake -f Makefile cmTC_55fa9/fast && /usr/bin/gmake  -f CMakeFiles/cmTC_55fa9.dir/build.make CMakeFiles/cmTC_55fa9.dir/build
gmake[1]: Verzeichnis „/home/tux/Schreibtisch/Git_SDL_Script/SDL/SDL3_ttf/build_win64/CMakeFiles/CMakeTmp“ wird betreten
Building C object CMakeFiles/cmTC_55fa9.dir/CheckSymbolExists.c.obj
/usr/bin/x86_64-w64-mingw32-gcc    -o CMakeFiles/cmTC_55fa9.dir/CheckSymbolExists.c.obj -c /home/tux/Schreibtisch/Git_SDL_Script/SDL/SDL3_ttf/build_win64/CMakeFiles/CMakeTmp/CheckSymbolExists.c
/home/tux/Schreibtisch/Git_SDL_Script/SDL/SDL3_ttf/build_win64/CMakeFiles/CMakeTmp/CheckSymbolExists.c: In function ‘main’:
/home/tux/Schreibtisch/Git_SDL_Script/SDL/SDL3_ttf/build_win64/CMakeFiles/CMakeTmp/CheckSymbolExists.c:8:19: error: ‘__GLIBC__’ undeclared (first use in this function); did you mean ‘__LINE__’?
    8 |   return ((int*)(&__GLIBC__))[argc];
      |                   ^~~~~~~~~
      |                   __LINE__
/home/tux/Schreibtisch/Git_SDL_Script/SDL/SDL3_ttf/build_win64/CMakeFiles/CMakeTmp/CheckSymbolExists.c:8:19: note: each undeclared identifier is reported only once for each function it appears in
gmake[1]: *** [CMakeFiles/cmTC_55fa9.dir/build.make:78: CMakeFiles/cmTC_55fa9.dir/CheckSymbolExists.c.obj] Fehler 1
gmake[1]: Verzeichnis „/home/tux/Schreibtisch/Git_SDL_Script/SDL/SDL3_ttf/build_win64/CMakeFiles/CMakeTmp“ wird verlassen
gmake: *** [Makefile:127: cmTC_55fa9/fast] Fehler 2


File /home/tux/Schreibtisch/Git_SDL_Script/SDL/SDL3_ttf/build_win64/CMakeFiles/CMakeTmp/CheckSymbolExists.c:
/* */
#include <stdlib.h>

int main(int argc, char** argv)
{
  (void)argv;
#ifndef __GLIBC__
  return ((int*)(&__GLIBC__))[argc];
#else
  (void)argc;
  return 0;
#endif
}