configure.ac: fix cairo-glesv2 help text
configure.ac: fix cairo-glesv2 help text

Finally figured out why --enable-cairo-gles2 wasn't working like
configure --help said it should.

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