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