The script currently assumes that srcdir is relative to builddir,
which is not the case for build directories outside the source tree
like the one used by jhbuild.
While we could get fancy and prepend the builddir if srcdir is relative,
there's nothing wrong with a relative path as we cd into the builddir
before evaluating srcdir.

https://bugzilla.gnome.org/show_bug.cgi?id=783206