Fixed GCC warning

glm/core/_detail.hpp

@@ -18,7 +18,7 @@ namespace detail
 {
 	class thalf;
 
-#if(__STDC_VERSION__ >= 199901L) // C99 detected, 64 bit types available
+#if(__STDC_VERSION__ && (__STDC_VERSION__ >= 199901L)) // C99 detected, 64 bit types available
 	typedef int64_t								sint64;
 	typedef uint64_t							uint64;
 #elif(GLM_COMPILER & GLM_COMPILER_VC)