Use llvm-config-3.x if llvm-config is not found

Ubuntu users may install a version specific llvm, so that llvm-config
looks like llvm-config-3.4, say.  Thus, if llvm-config doesn't exist,
we try llvm-config-3.5 down to -3.2 in order.

Note that since there does not seem to be a clang equivalent of
llvm-config, this may not solve all problems.

Fix by Kevin Tew <tew@byu.edu>.