Link against clang/llvm as late as possible

This allows the static libraries to be linked into an dynamic llvm compiler pass library without causing runtime link errors on Mac OS X due to doubly defined symbols.

Unfortunately, I can currently *only* test on Mac OS X, where this works. Hence, this needs to be tested on Linux before being committed.

I dropped the target_link_libraries that were not depending on any other libraries; maybe those should have been kept. I happily update the patch if this is required.