build: fix EGL build when no X11 headers are present

build: fix EGL build when no X11 headers are present

eglplatform.h defaults to X11 on Unix unless told otherwise, so if we're doing a
build without any X11 support tell it so that we don't try including headers
that don't exist.

Also set GL_PC_FLAGS so that the definition is in egl.pc, so that applications
using EGL don't try to pull in X11 headers on systems where EGL was configured
without X11 support.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=64959
Signed-off-by: Ross Burton <ross.burton@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>