Define GLIB_CFLAGS and GTK_CFLAGS variables in config.nmake (based on

Define GLIB_CFLAGS and GTK_CFLAGS variables in config.nmake (based on
variables the user configures - the user isn't expected to change
GLIB_CFLAGS or GTK_CFLAGS, and there's a comment nothing that users
shouldn't have to do so), which contain the appropriate "/I" flags for
building stuff that requires only GLib, and stuff that required GTK+ and
GLib, respectively, and use those macros in the Makefile.nmake files.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@7884 f5534014-38df-0310-8fa8-9805f1628bb7