configure.ac: bump to version 1.8.0 for the official release
configure.ac: bump to version 1.8.0 for the official release

kwm81/weston,kwm81/weston,jonnylamb/weston,giucam/weston,xorgy/weston,mchalupa/weston,giucam/weston,Tarnyko/weston-xdg_surface_present,eyolfson/weston,mchalupa/weston,Fantu/compositor-spice,eyolfson/weston,xorgy/weston,jonnylamb/weston,krezovic/weston,sir-murray/weston,udoprog/weston,krezovic/weston,sir-murray/weston,Fantu/compositor-spice,Gnurou/weston,Gnurou/weston,udoprog/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present