Try to improve options.mk and update buildlink3.mk and Makefile.common
accordingly:
 o add an "opengl" option (ATM disabled by default) to enable GLX, WebGL
   and accelerated composite support
 o add an "enchant" option to enable spellcheck support (enabled by default,
   like www/webkit-gtk)
Update CONFIGURE_ARGS to the current version:
 o --with-target=x11 -> --enable-x11-target
 o --with-font-backend no more supported