Adjust Darwin build to use the default 'two level namespace' linking

Adjust Darwin build to use the default 'two level namespace' linking
method.  Fix a number of places where shared libraries were linked without
mentioning all the libraries they depend on; the Darwin and AIX ports
are known to require this, and it doesn't seem to hurt any other supported
platforms.  (Hence, remove code in pl/tcl makefile that tried to avoid
mentioning other libs if not needed.)