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.
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