cmake: avoid copy of headers by moving generated headers to include/build_config (thanks @madebr!)

Shove all build system generated headers in include/build_config.
Doing this, cmake does not need to copy the headers anymore.
This was done for external projects vendoring SDL.

See https://github.com/libsdl-org/SDL/issues/6641#issuecomment-1328092535

include/build_config/SDL_revision.h.cmake

@@ -0,0 +1,8 @@
+#cmakedefine SDL_VENDOR_INFO "@SDL_VENDOR_INFO@"
+#define SDL_REVISION_NUMBER 0
+
+#ifdef SDL_VENDOR_INFO
+#define SDL_REVISION "@SDL_REVISION@ (" SDL_VENDOR_INFO ")"
+#else
+#define SDL_REVISION "@SDL_REVISION@"
+#endif