DEVTOOLS: Replace ugly SDL/MinGW hack with a cleaner hack

Instead of having main=SDL_main defined by pkg-config/sdl2-config --cflags
and undefining it everywhere, strip it in its origin.

devtools/create_cryomni3d/create_cryomni3d_dat.cpp

@@ -23,12 +23,6 @@
 // Disable symbol overrides so that we can use system headers.
 #define FORBIDDEN_SYMBOL_ALLOW_ALL
 
-// HACK to allow building with the SDL backend on MinGW
-// see bug #3412 "TOOLS: MinGW tools building broken"
-#ifdef main
-#undef main
-#endif // main
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <assert.h>