Add full support for -rpath/-R, both in shared libraries and

applications, at least on the platforms where it's known how
to do it.

Note: this has only been tested on GNU-based platforms (Linux), and
needs to be tested on all others.  Additionally, it's not yet
supported on the following platforms, for lack of information:

Darwin (MacOS X)
Cygwin
OSF1/Alpha
SVR3
ReliantUNIX

Please help out with testing and the platforms we don't yet know well
enough.