gallivm: Disable gallivm to fix build with LLVM 3.6

gallivm: Disable gallivm to fix build with LLVM 3.6

LLVM commit r218316 removes the JITMemoryManager class, which is
the parent for a seemingly important class in gallivm.  In order to
fix the build, I've wrapped most of lp_bld_misc.cpp in
if HAVE_LLVM < 0x0306 and modifyed the
lp_build_create_jit_compiler_for_module() function to return false
for 3.6 and newer which effectively disables the gallivm functionality.

I realize this is overkill, but I could not come up with a simple
solution to fix the build.  Also, since 3.6 will be the first release
without the old JIT, it would be really great if we could
move gallivm to use the C API only for accessing MCJIT.  There
is still time before the 3.6 release to extend the C API in
case it is missing some functionality that is required by gallivm.