numpy.h: minor preprocessor fix suggested by @chaekwan

Author: wjakob

include/pybind11/numpy.h

@@ -311,7 +311,7 @@ template <typename T> using remove_all_extents_t = typename array_info<T>::type;
 
 template <typename T> using is_pod_struct = all_of<
     std::is_standard_layout<T>,     // since we're accessing directly in memory we need a standard layout type
-#if !defined(__GNUG__) || defined(_LIBCPP_VERSION) || defined(_GLIBCXX_USE_CXX11_ABI)
+#if !defined(__GNUG__) || defined(_LIBCPP_VERSION) || (defined(_GLIBCXX_USE_CXX11_ABI) && _GLIBCXX_USE_CXX11_ABI != 0)
     // _GLIBCXX_USE_CXX11_ABI indicates that we're using libstdc++ from GCC 5 or newer, independent
     // of the actual compiler (Clang can also use libstdc++, but it always defines __GNUC__ == 4).
     std::is_trivially_copyable<T>,