Choose correct libdir in Makefile, pkgconfig etc

[barathrm]

The Makefile should let the user configure the libdir path to allow installing the library to for instance /usr/lib64 instead of /usr/lib. This should also set the correct path in the etebase.pc file.

This makes it easier and less hacky to package libetebase for distros.

I think we've already discussed this, but creating an issue to track this until we've had the time to fix this and make a new version.

[tasn]

Happy for this to be added. I vaguely remember we talked about it, but I thought it was already fixed tbh. :P