Set two (resp. three) GAP roots for out of tree builds

[fingolfin]

Right now, a normal (non-HPC) build always sets a single GAP root, pointing to the srcdir. But this way, make bootstrap won't work for out-of-tree builds, because the packages end up in builddir. To solve this, we should set two root directories for out-of-tree builds: First the builddir, then the srcdir.

For HPC-GAP build, that would be three GAP roots: two for the srcdir, and on for the builddir.

[fingolfin]

And once this is implemented, remove the hack in .travis.yml, i.e. remove

if [[ $BUILDDIR != . ]] ; then mv pkg $GAPROOT/ ; fi

[fingolfin]

The simple (?) solution for this should be to modify bin/gap.sh suitably.