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.