Allow the Gtk+ and Qt versions to be built simultaneously (with autotools).

Allow the Gtk+ and Qt versions to be built simultaneously (with autotools).

./configure now accepts:
	--with-gtk2 :  enabled by default; exclusive of --with-gtk3
	--with-gtk3 : disabled by default; exclusive of --with-gtk2
	--with-qt   : disabled by default, can be specified with gtk
	--enable-wireshark : controls whether *any* GUI is built

If Qt is enabled then a new program is created: "wireshark-qt".  "wireshark"
remains the Gtk+ version.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@50147 f5534014-38df-0310-8fa8-9805f1628bb7