gallivm: Make MCJIT a runtime option.
gallivm: Make MCJIT a runtime option.

On the LLVM versions that support it, so we can easily switch between
MCJIT/old-jit for testing.

The new option is GALLIVM_MCJIT.

Unfortunately setting GALLIVM_MCJIT=1 for LLVM 3.3 or 3.4 causes
segfault, both on Linux and Windows.  I'm almost certain this used to
work, so there probably is a regression somewhere.

Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler