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 <cd36ec7a67dfb8d589bc66b85fd0a6cf213f3854@intel.com>
Reviewed-by: Chad Versace <a557f7f75bf28dc19ac99107f5cc7cd6b3fbdb1a@linux.intel.com>
Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

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