package/mesa3d: add a new option to enable GLX support

package/mesa3d: add a new option to enable GLX support

When switching to meson build system, enabling GLX support
require at least one DRI or Gallium dirver.

So add a dependency on BR2_PACKAGE_MESA3D_DRIVER for
BR2_PACKAGE_MESA3D_OPENGL_GLX option.

quoting the meson build system:
"dri based GLX requires at least one DRI driver." [1]
"Even when building just gallium drivers the user probably wants dri." [2]

[1] https://cgit.freedesktop.org/mesa/mesa/tree/meson.build?h=19.0#n340
[2] https://cgit.freedesktop.org/mesa/mesa/tree/meson.build?h=19.0#n266

Signed-off-by: Romain Naour <romain.naour@smile.fr>
[Bernd: moved x11 dependencies from glx condition to xorg7 condition to
 fix build error with enabled egl without glx
 BR2_PACKAGE_HAS_LIBGL needs to depend on BR2_PACKAGE_MESA3D_OPENGL_GLX]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Arnout: small changes to Config.in help text and menus]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>