mtools: fix target compilation

It needs some help regarding setpgrp, fixes:
http://autobuild.buildroot.net/results/33e/33e38bf1c30cc09dbe992dfb95c48997ca860212/


Also disable X11 support since it picks up the host variant.
And use normal indentation for variables.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>