configs: Always use -fPIC for shared libraries, never for static

configs: Always use -fPIC for shared libraries, never for static

Mesa currently disables -fPIC for DRI on x86, but most Linux distros are
re-enabling -fPIC for all DRI arches. Let's just do that here since
that's normally what's wanted for shared libraries. Some justification:

http://bugs.gentoo.org/show_bug.cgi?id=110840#c9

On the other hand, position-independent code is only necessary when
building shared libraries, so disable it for the static cases.