OSX: Allow an empty INSTALL_NAME_DIR to override MACOSX_RPATH.

With this, projects can enable MACOSX_RPATH by default, but still have
a way to install libraries with no install name dirs into locations
such as /usr/local/lib by setting INSTALL_NAME_DIR="".
Merge topic 'rpath-on-mac' into next

78cd884 OSX: Allow an empty INSTALL_NAME_DIR to override MACOSX_RPATH.