Merge pull request #325 from past-due/trivially_copyable

Allow defining BASISU_HAVE_STD_TRIVIALLY_COPYABLE

Author: richgel999

transcoder/basisu_containers.h

@@ -188,7 +188,7 @@ namespace basisu
 
 #define BASISU_IS_SCALAR_TYPE(T) (scalar_type<T>::cFlag)
 
-#if defined(__GNUC__) && __GNUC__<5
+#if !defined(BASISU_HAVE_STD_TRIVIALLY_COPYABLE) && defined(__GNUC__) && __GNUC__<5
    #define BASISU_IS_TRIVIALLY_COPYABLE(...) __has_trivial_copy(__VA_ARGS__)
 #else
    #define BASISU_IS_TRIVIALLY_COPYABLE(...) std::is_trivially_copyable<__VA_ARGS__>::value