package/mesa3d: Remove libGL headers/pkgconfig when OpenGL is disabled

mesa3d defaults to --enable-opengl due to its weird configure.ac:
http://git.buildroot.net/buildroot/commit/package/mesa3d/mesa3d.mk?id=384d601d4ea2c41bb7928f756bfe88af4d38b38b

This leads to include/GL/*.h and lib/pkgconfig/gl.pc being installed to
STAGING_DIR. These files are enough for other packages to wrongly assume
that libGL.so is present, like vlc.

Fixes
http://autobuild.buildroot.net/results/e63/e639dabfb260006acb8a204851b96f9275ce53a7/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>