include: Fix symbols in public headers that start with an underscore.

Fixes #7352.

include/begin_code.h

@@ -28,10 +28,10 @@
  */
 
 /* This shouldn't be nested -- included it around code only. */
-#ifdef _begin_code_h
+#ifdef SDL_begin_code_h
 #error Nested inclusion of begin_code.h
 #endif
-#define _begin_code_h
+#define SDL_begin_code_h
 
 #ifndef SDL_DEPRECATED
 #  if defined(__GNUC__) && (__GNUC__ >= 4)  /* technically, this arrived in gcc 3.1, but oh well. */
@@ -171,17 +171,17 @@
 #define SDL_FALLTHROUGH [[fallthrough]]
 #else
 #if defined(__has_attribute)
-#define _HAS_FALLTHROUGH __has_attribute(__fallthrough__)
+#define SDL_HAS_FALLTHROUGH __has_attribute(__fallthrough__)
 #else
-#define _HAS_FALLTHROUGH 0
+#define SDL_HAS_FALLTHROUGH 0
 #endif /* __has_attribute */
-#if _HAS_FALLTHROUGH && \
+#if SDL_HAS_FALLTHROUGH && \
    ((defined(__GNUC__) && __GNUC__ >= 7) || \
     (defined(__clang_major__) && __clang_major__ >= 10))
 #define SDL_FALLTHROUGH __attribute__((__fallthrough__))
 #else
 #define SDL_FALLTHROUGH do {} while (0) /* fallthrough */
-#endif /* _HAS_FALLTHROUGH */
-#undef _HAS_FALLTHROUGH
+#endif /* SDL_HAS_FALLTHROUGH */
+#undef SDL_HAS_FALLTHROUGH
 #endif /* C++17 or C2x */
 #endif /* SDL_FALLTHROUGH not defined */

include/close_code.h

@@ -26,10 +26,10 @@
  *  after you finish any function and structure declarations in your headers
  */
 
-#ifndef _begin_code_h
+#ifndef SDL_begin_code_h
 #error close_code.h included without matching begin_code.h
 #endif
-#undef _begin_code_h
+#undef SDL_begin_code_h
 
 /* Reset structure packing at previous byte alignment */
 #if defined(_MSC_VER) || defined(__MWERKS__) || defined(__BORLANDC__)