[Buildsystem] Honor --buiddir option for dunecontrol again.

In one of the latest patches I introduced a bug which caused
the --builddir option of dunecontrol to not make any effect.
This patch fixes this.
Fixes indentation.
[Buildsystem] Allow specification of USE_CMAKE in opts file.

With this patch it is possible to request CMake for building
by setting USE_CMAKE=yes in an option file.