Build failure against Glib 2.68.3

[danfe]

Builds fine against Glib version 2.66.8, but fails against 2.68.3 with the following errors:

In file included from fontcairo.cpp:20:
In file included from /usr/local/include/pango-1.0/pango/pangocairo.h:25:
In file included from /usr/local/include/pango-1.0/pango/pango.h:25:
In file included from /usr/local/include/pango-1.0/pango/pango-attributes.h:25:
In file included from /usr/local/include/pango-1.0/pango/pango-font.h:25:
In file included from /usr/local/include/pango-1.0/pango/pango-coverage.h:25:
In file included from /usr/local/include/glib-2.0/glib-object.h:22:
In file included from /usr/local/include/glib-2.0/gobject/gbinding.h:28:
In file included from /usr/local/include/glib-2.0/glib.h:32:
In file included from /usr/local/include/glib-2.0/glib/gasyncqueue.h:32:
In file included from /usr/local/include/glib-2.0/glib/gthread.h:32:
In file included from /usr/local/include/glib-2.0/glib/gatomic.h:31:
/usr/include/c++/v1/type_traits:517:57: error: reference to 'std' is ambiguous
/usr/include/c++/v1/type_traits:517:81: error: use of template template parameter '_Templ' requires template arguments
using _IsValidExpansion _LIBCPP_NODEBUG_TYPE = decltype(std::__sfinae_test_impl<_Templ, _Args...>(0));
...
/usr/include/c++/v1/type_traits:1404:27: error: '_Tp' does not refer to a value
decltype(_VSTD::__declval<_Tp>(0))
...
/usr/include/c++/v1/type_traits:2563:66: error: '_Tp1' does not refer to a value
        typename __is_destructible_apply<decltype(_VSTD::declval<_Tp1&>().~_Tp1(
))>::type

[danfe]

Attached patch fixes the build for me, but I'm not sure whether this is the right approach.