Fixed gcc warnings for apps using SDL headers with -Wundef flag.

Fixes Bugzilla #1216.

Thanks to Dimitris Zenios for the patch!

include/SDL_assert.h

@@ -53,7 +53,7 @@ assert can have unique static variables associated with it.
 /* Don't include intrin.h here because it contains C++ code */
 extern void __cdecl __debugbreak(void);
     #define SDL_TriggerBreakpoint() __debugbreak()
-#elif (defined(__GNUC__) && ((__i386__) || (__x86_64__)))
+#elif (defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__)))
     #define SDL_TriggerBreakpoint() __asm__ __volatile__ ( "int $3\n\t" )
 #elif defined(HAVE_SIGNAL_H)
     #include <signal.h>
@@ -63,7 +63,7 @@ extern void __cdecl __debugbreak(void);
     #define SDL_TriggerBreakpoint()
 #endif
 
-#if (__STDC_VERSION__ >= 199901L) /* C99 supports __func__ as a standard. */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) /* C99 supports __func__ as a standard. */
 #   define SDL_FUNCTION __func__
 #elif ((__GNUC__ >= 2) || defined(_MSC_VER))
 #   define SDL_FUNCTION __FUNCTION__

include/SDL_atomic.h

@@ -140,7 +140,7 @@ void _ReadWriteBarrier(void);
 /* Platform specific optimized versions of the atomic functions,
  * you can disable these by defining SDL_DISABLE_ATOMIC_INLINE
  */
-#if SDL_ATOMIC_DISABLED
+#if defined(SDL_ATOMIC_DISABLED) && SDL_ATOMIC_DISABLED
 #define SDL_DISABLE_ATOMIC_INLINE
 #endif
 #ifndef SDL_DISABLE_ATOMIC_INLINE