patch library load paths if they are incomplete

Only on OS X.

link-time and run-time library paths can differ, causing `--zmq=/some/prefix` to successfully link, but not produce an executable that can be used without DYLD_LIBRARY_PATH, depending on how the linked library is compiled.

This patches any library linked as simply 'libfoo.dylib', and replaces the path with '/full/path/to/libfoo.dylib', so that it will load correction using `install_name_tool -change`.