Don't try to create a temp install without abs_top_builddir.

Don't try to create a temp install without abs_top_builddir.

Otherwise, we effectively act as if abs_top_builddir were the root
directory, which is quite dangerous if the user happens to have
permissions to do things there.  This can crop up in PGXS builds,
for example.

Report by Sandro Santilli, patch by me, review by Noah Misch.