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.
linux-dri. Turn on -O by default (-O2 provides a slight improvement in
performance, at a large cost to debuggability). Turn off -fPIC on x86 by
default. Turn on -Wall, to catch more stupid mistakes. This could be both done
cleaner, and done for more config files, but it works for me for now.