package/mc: Fix xlib_libX11 dependency

On Peter´s autobuilder building mc fails:
http://autobuild.buildroot.net/?reason=mc-4.8.14

The reason can be found in
http://autobuild.buildroot.net/results/e25/e25587c17c2e8ddae83f9ed02541931f510bfcd6//build-end.log

checking for X... libraries /usr/lib, headers /usr/include

where configure picks up xorg libraries although no x11 stuff is enabled in the
defconfig used for this build.

This leads to subsequent errors visible in
http://autobuild.buildroot.net/results/e25/e25587c17c2e8ddae83f9ed02541931f510bfcd6//mc-4.8.14/config.log

mips64el-ctng_n64-linux-gnu-gcc: ERROR: unsafe header/library path used in cross-compilation: '/usr/lib'

causing most configure checks to fail afterwards. To fix the problem the mc
package should honour an optional dependency to the xlib_libX11 package.

Fixes
http://autobuild.buildroot.net/results/e25/e25587c17c2e8ddae83f9ed02541931f510bfcd6/
http://autobuild.buildroot.net/results/b7a/b7a2b69c53e91e0173b7f0c241d7e06b4382c52a/
http://autobuild.buildroot.net/results/805/805675236190f7f5f42d498f4a9f9bf0e1bab648/


and others

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>