configure.ac: Only enable enable_simple_egl_clients if enable_egl is yes
configure.ac: Only enable enable_simple_egl_clients if enable_egl is yes

This simplifies a few checks and makes sure we only build the
subsurfaces client if EGL is enabled.

https://bugs.freedesktop.org/show_bug.cgi?id=68727

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