include: Move SDL_HAS_BUILTIN to SDL_begin_code.h, document it.

This macro fits better here, since most of the other macros in this file are
for testing similar compiler/target features, etc.

include/SDL3/SDL_stdinc.h

@@ -147,17 +147,6 @@ void *alloca(size_t);
        typedef int SDL_compile_time_assert_ ## name[(x) * 2 - 1]
 #endif
 
-/**
- * Check if the compiler supports a given builtin.
- * Supported by virtually all clang versions and recent gcc. Use this
- * instead of checking the clang version if possible.
- */
-#ifdef __has_builtin
-#define SDL_HAS_BUILTIN(x) __has_builtin(x)
-#else
-#define SDL_HAS_BUILTIN(x) 0
-#endif
-
 /**
  * The number of elements in a static array.
  *