configure: error out when building GLX w/o the X11 platform

configure: error out when building GLX w/o the X11 platform

Building EGL/Vulkan/other without X11, while GLX is enabled is confusing
and misleading. In practise anyone aiming at the former will also
disable GLX.

The inverse (some examples below) should still work:
 ./configure --disable-glx --with-platforms=x11 --with-vulkan-drivers=intel
 ./configure --disable-glx --with-platforms=x11 --enable-egl

Keep in mind that the X11 platform is enabled, by default.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>