mesa3d-demos: select libglew/libglu if needed

Fixes:
http://autobuild.buildroot.net/results/4b1/4b16c9797c653c248725a55d9b8eafc4634b968c/build-end.log



libglew/libglu are added to _DEPENDENCIES for X11+GL, but not selected at
the kconfig level.

While we're at it, correct the dependencies for the X11 selects as well
(only used if GL is available).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>