configure fails if requirement can't be met for user's explicit request

configure fails if requirement can't be met for user's explicit request

Notify user an error if user provides --enable-x11/--enable-glx/--enable-egl/--enable-wayland
however the requirement can't be met. drm has been checked mandatorily
in the script

v2: Remove XEXT_CFLAGS/XFIXES_CFLAGS from va/x11/Makefile.am and use
$X11_PKG_ERRORS in the error message if the requirement is met for VA/X11

This fixes https://github.com/01org/libva/issues/68

Signed-off-by: Xiang, Haihao <haihao.xiang@intel.com>
Reviewed-by: U. Artie Eoff <ullysses.a.eoff@intel.com>