Support "make check" for PGXS extensions

This is a quick follow-up to the extension_control_path patch. With
this little additional patch, you can now run "make check" in PGXS-using
extensions (instead of having to do make install; make installcheck with
a running instance). I think this would be very convenient for
extension development.

The patch is still rough, probably needs a bit of work to do proper
escaping, quoting, further testing, and it will probably break if you
use a different source tree layout. Maybe with a bit of help we can get
this robust enough. Or otherwise, it can at least serve as inspiration
for what you can implement yourself in your extension's makefile.