Refinement to GCC 4 workaround

Require either a trivial copy constructor or a trivial copy assignment,
but not both. This is slightly more relaxed than being
TriviallyCopyable, which requires that both are either trivial or
deleted, and that at least one is not deleted. With this check, one of
them could be user-provided.

include/pybind11/numpy.h

@@ -250,8 +250,7 @@ template <typename T> using is_pod_struct = all_of<
 #else
     // GCC 4 doesn't implement is_trivially_copyable, so approximate it
     std::is_trivially_destructible<T>,
-    std::has_trivial_copy_constructor<T>,
-    std::has_trivial_copy_assign<T>,
+    satisfies_any_of<T, std::has_trivial_copy_constructor, std::has_trivial_copy_assign>,
 #endif
     satisfies_none_of<T, std::is_reference, std::is_array, is_std_array, std::is_arithmetic, is_complex, std::is_enum>
 >;