libepoxy: make EGL support optional

This commit adds a patch to the libepoxy package to make the EGL
support optional, which allows libepoxy to build with a pure OpenGL
Mesa3D configuration (i.e without EGL/OpenGLES).

Fixes:

  http://autobuild.buildroot.net/results/88774af2845e17cab021a72c8f3171fe30b3a1ff/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>