Make IS_TRIVIALLY_CONSTRUCTIBLE consistent on GCC < 5

std::is_trivially_constructible<T> is equivalent to std::is_trivially_default_constructible<T>
std::has_trivial_default_constructor<T> is the GCC < 5 name for std::is_trivially_default_constructible<T>

std::is_trivial was also used when compiling with clang, due to clang's use of __GNUC__. Test __clang__
to target the intended implementations.

src/compat.h

@@ -14,10 +14,10 @@
 
 // GCC 4.8 is missing some C++11 type_traits,
 // https://www.gnu.org/software/gcc/gcc-5/changes.html
-#if defined(__GNUC__) && __GNUC__ < 5
-#define IS_TRIVIALLY_CONSTRUCTIBLE std::is_trivial
+#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 5
+#define IS_TRIVIALLY_CONSTRUCTIBLE std::has_trivial_default_constructor
 #else
-#define IS_TRIVIALLY_CONSTRUCTIBLE std::is_trivially_constructible
+#define IS_TRIVIALLY_CONSTRUCTIBLE std::is_trivially_default_constructible
 #endif
 
 #ifdef WIN32