autoconf: Attempt to figure out the PIC flags for the platform

autoconf: Attempt to figure out the PIC flags for the platform

This commit adds an autoconf macro, MESA_PIC_FLAGS, which sets the
PIC flags according to platform and static/shared setting. The platform
specifics are taken straight from libtool.m4 and stripped down to just
the flags and platforms we cover in Mesa. This should hopefully make it
possible to use autoconf on non-GCC platforms.

The macro is added external to configure.ac in acinclude.m4 since it's
pretty bloated.

Note to BSDers: Previously, x86 defaulted to non-PIC on FreeBSD. I
didn't carry that preference into this macro. Instead, you can just use
--disable-pic where desired.