component/libepoxy: add to build gtk-3.

component/libepoxy/buildout.cfg

+[buildout]
+extends =
+  ../mesa/buildout.cfg
+  ../pkgconfig/buildout.cfg
+  ../xorg/buildout.cfg
+  ../xz-utils/buildout.cfg
+
+[libepoxy]
+recipe = slapos.recipe.cmmi
+url = https://github.com/anholt/libepoxy/releases/download/v1.4/libepoxy-1.4.0.tar.xz
+md5sum = d8d8cbf2beb64975d424fcc5167a2a38
+environment =
+  PATH=${pkgconfig:location}/bin:${xz-utils:location}/bin:%(PATH)s
+  PKG_CONFIG_PATH=${mesa:location}/lib/pkgconfig:${libX11:location}/lib/pkgconfig:${libX11:pkg_config_depends}:${libXext:location}/lib/pkgconfig:${xdamage:location}/lib/pkgconfig:${damageproto:location}/lib/pkgconfig:${xfixes:location}/lib/pkgconfig:${fixesproto:location}/lib/pkgconfig