Added gtk3 to autotools (fixed compile time issue)

Added gtk3 to autotools (fixed compile time issue)
	modified:   Makefile
	modified:   aclocal.m4
	modified:   config.status
	modified:   configure
	modified:   configure.in
	modified:   intl/Makefile
	modified:   po/Makefile
	modified:   po/Makefile.in
	modified:   po/fr.gmo
	modified:   po/fr.po
	modified:   po/gtkdiskfree.pot
	modified:   src/Makefile
	modified:   src/about.c
	modified:   src/config.h
	modified:   src/config.h.in
	modified:   src/icons/Makefile
	modified:   src/interface.c
	modified:   src/menus.c
	modified:   src/widgets.c