Fix check_stdlib_usage.py libc call matching

It now matches libc usage inside statements,
and skips libc-like usage in strings or as struct members.

src/main/emscripten/SDL_sysmain_runapp.c

@@ -31,7 +31,7 @@ EM_JS_DEPS(sdlrunapp, "$dynCall,$stringToNewUTF8");
 // even though we reference the C runtime's free() in other places, it appears
 // to be inlined more aggressively in Emscripten 4, so we need a reference to
 // it here, too, so the inlined Javascript doesn't fail to find it.
-EMSCRIPTEN_KEEPALIVE void force_free(void *ptr) { free(ptr); }
+EMSCRIPTEN_KEEPALIVE void force_free(void *ptr) { free(ptr); } // This should NOT be SDL_free()
 
 int SDL_RunApp(int argc, char *argv[], SDL_main_func mainFunction, void * reserved)
 {