Internal version is an LLVM 2.8 with 2 patches backported from LLVM 2.9 to fix a
crash on AVX chips.
So drop support for building with external LLVM 2.8, and add support for
building with external LLVM 2.9 instead.
Caveat:
stack smashing protection is broken on LLVM 2.9 so it is disabled
Example on Debian:
apt-get install llvm-2.9-dev
./configure --enable-llvm --with-system-llvm=/usr/bin/llvm-config-2.9
With -m32 LLVM gets the target arch right, and configure gets it wrong.
LLVM's detection always overrides the arch, so don't warn on these mismatches.
startup.cbc will get the correct arch when JIT is available, which is the only
case where the arch matters anyway.
When there is no JIT the arch reported to startup.cbc will be configure's which might be wrong.
But since there is no JIT startup.cbc shouldn't need to check the arch anyway.
It will still be able to detect sparc vs x86 though, not 32 vs 64-bit.
