A clean patch

Now it should be much cleaner. The commit aims to remove the test from configure.ac and move the printing to the echo-install-dirs script which can print out the paths without tests. I think this solution is more bullet proof.

Also, the printing of the message about the pkgconfig path is made unconditional because it is easier as we don't know what other paths the user is installing to. It could be made conditional again if you think that is better.