gen-introspect: pretend to be lint

Some system headers (notably <stdarg.h> on FreeBSD) get upset when they
see an unexpected compiler.  We go out of our way to pass -U__GNUC__ to
cpp from the scanner code during gen-introspect and this is causing
problems.

As a workaround, we can pretend to be lint, which fixes the issue at
least for FreeBSD.

https://bugzilla.gnome.org/show_bug.cgi?id=722954
vala.m4: honour CC, CPPFLAGS, CFLAGS, LDFLAGS

These variables may be set to values that are required to build
executables.  Make sure we honour them from vala.m4 when attempting to
check for packages.

https://bugzilla.gnome.org/show_bug.cgi?id=724176
gtk+-3.0: Update from git
glib-2.0: add missing symbols from 2.38
glib-2.0: add new symbols for 2.40
gio-2.0: update to 2.39.5
gio-2.0: Update to 2.39.5+
gio-2.0: fix add_main_option_entries
gedit: deprecate in favor of bindings distributed by gedit.

See bug #724172.
gtk+-3.0: Update to 3.11.6
Release 0.23.3