Always define SDL_COMPILE_TIME_ASSERT as static_assert() in C++

Apparently some versions of gcc will define __STDC_VERSION__ even when compiling in C++ mode.

Fixes https://github.com/libsdl-org/SDL/issues/6078

include/SDL_stdinc.h

@@ -373,14 +373,22 @@ typedef uint64_t Uint64;
 #endif
 #endif /* SDL_DISABLE_ANALYZE_MACROS */
 
-#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)
-#define SDL_COMPILE_TIME_ASSERT(name, x) _Static_assert(x, #x)
-#elif defined(__cplusplus) && (__cplusplus >= 201103L)
+#ifndef SDL_COMPILE_TIME_ASSERT
+#if defined(__cplusplus)
+#if (__cplusplus >= 201103L)
 #define SDL_COMPILE_TIME_ASSERT(name, x)  static_assert(x, #x)
-#else /* universal, but may trigger -Wunused-local-typedefs */
+#endif
+#elif defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)
+#define SDL_COMPILE_TIME_ASSERT(name, x) _Static_assert(x, #x)
+#endif
+#endif /* !SDL_COMPILE_TIME_ASSERT */
+
+#ifndef SDL_COMPILE_TIME_ASSERT
+/* universal, but may trigger -Wunused-local-typedefs */
 #define SDL_COMPILE_TIME_ASSERT(name, x)               \
        typedef int SDL_compile_time_assert_ ## name[(x) * 2 - 1]
 #endif
+
 /** \cond */
 #ifndef DOXYGEN_SHOULD_IGNORE_THIS
 SDL_COMPILE_TIME_ASSERT(uint8, sizeof(Uint8) == 1);