Add LLVM libdir to library search path in out-of-tree builds.

This previously was not required because in an out-of-tree build Polly would
only build libraries (LLVMPolly, libPolly, libPollyISL, libPollyPPCG), but no
executables where the libraries would be linked to. This will change when adding
unittests in a follow-up commit.

llvm-svn: 279730