Use absolute paths for --vapidir

Use absolute paths for --vapidir

In the rules currently generated by Automake, valac currently runs
cd'd into the ${srcdir}. It seems unwise to rely on that, and the
only thing that will work regardless of whether ${srcdir} is ".",
relative or absolute is its "absolutized" version.

Similarly, look in both the ${srcdir} and the ${builddir} for Vala
bindings we generate: the ${srcdir} because that's where valac currently
puts them, and the ${builddir} because that's where they ought to go
in principle.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=694883
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>