Fix fakesdl headers when SDL directory included

We introduced the `fakesdl` headers back in emscripten-core#18443 so that anyone trying
to use SDL without using `-sUSE_SDL` (or similar) would see an error.

However we only covered the `SDL_xx.h` include case not the
`SDL/SDL_xx.h` use case.

This means that some users were not seeing the errors they should,
including a few of our test cases.

Split out from emscripten-core#26275

Author: sbc100

site/source/docs/tools_reference/settings_reference.rst

@@ -1476,11 +1476,12 @@ var LEGALIZE_JS_FFI = true;
 
 // Ports
 
-// Specify the SDL version that is being linked against.
-// 1, the default, is 1.3, which is implemented in JS
-// 2 is a port of the SDL C code on emscripten-ports
-// When AUTO_JS_LIBRARIES is set to 0 this defaults to 0 and SDL
-// is not linked in.
+// Specify the SDL version that is being linked against:
+//
+// - 0, the default, means no SDL headers or libraries will be used.
+// - 1 is a port of SDL 1.3, implemented in JS.
+// - 2 and 3 are upstream ports build from the official SDL codebase.
+//
 // Alternate syntax for using the port: --use-port=sdl2
 // [compile+link]
 var USE_SDL = 0;