attempt to compile against GTK+ 3.0 (which can be installed at the same
time as GTK+ 2.0). Also place a copy of the autoconf macro for finding
GTK+ 3.0 in the aclocal-fallback directory taken from the GTK+ 3.0
distribution.
(Note: Wireshark won't finish compiling with GTK+ 3.0 yet...)
svn path=/trunk/; revision=35918