stdinc: use __has_include to detect <stdbool.h>

Newer emscripten SDK does an unconditional #include <stdbool.h>,
which clashes with our custom bool.h typedef

include/SDL3/SDL_stdinc.h

@@ -45,6 +45,11 @@
 #endif
 
 #ifndef __cplusplus
+#if defined(__has_include) && !defined(SDL_INCLUDE_STDBOOL_H)
+#if __has_include(<stdbool.h>)
+#define SDL_INCLUDE_STDBOOL_H
+#endif
+#endif
 #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L) || \
     (defined(_MSC_VER) && (_MSC_VER >= 1910 /* Visual Studio 2017 */)) || \
     defined(SDL_INCLUDE_STDBOOL_H)