[dunecontrol] support out-of-tree builds

up to now you could specify a BUILDDIR variable, which implied that modules were built in
$srdir/$BUILDDIR.
Imagine you have your dune modules in $HOME/Src. When you set BUILDDIR=build.g++
your dune-common module is built in $HOME/Src/dune-common/build.g++
Now you change BUILDDIR to an absolute path, e.g. BUILDDIR=$HOME/Build.g++
With the latest change dunecontrol will now build dune-common in
$HOME/Build.g++/dune-common/

Thanks to Angar for bugging me :-)