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_begin_code.h

@@ -262,10 +262,31 @@
  */
 #define SDL_RESTRICT __restrict__
 
+/**
+ * Check if the compiler supports a given builtin functionality.
+ *
+ * This allows preprocessor checks for things that otherwise might fail
+ * to compile.
+ *
+ * Supported by virtually all clang versions and more-recent GCCs. Use this
+ * instead of checking the clang version if possible.
+ *
+ * On compilers without has_builtin support, this is defined to 0 (always false).
+ *
+ * \since This macro is available since SDL 3.1.3.
+ */
+#define SDL_HAS_BUILTIN(x) __has_builtin(x)
+
 /* end of wiki documentation section. */
 #endif
 
-
+#ifndef SDL_HAS_BUILTIN
+#ifdef __has_builtin
+#define SDL_HAS_BUILTIN(x) __has_builtin(x)
+#else
+#define SDL_HAS_BUILTIN(x) 0
+#endif
+#endif
 
 #ifndef SDL_DEPRECATED
 #  if defined(__GNUC__) && (__GNUC__ >= 4)  /* technically, this arrived in gcc 3.1, but oh well. */

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.
  *