Last of the patches from pypa#73 
Might close pypa/setuptools#3257

Dual purposes here:
- On platforms like Cygwin that don't have `rpath`, try to avoid adding things to `rpath`
- Some distribution binary package makers require that no shared library list a system library directory (`/lib`, `/lib64`, `/usr/lib`, `/usr/lib64`) in its `rpath`; this patch simplifies the code to ensure the shared library can find its dependencies at runtime.

Avoids problems with odd LIBDIR
Package managers will be putting things in LIBDIR anyway,
so this should catch all use-cases I know of.

The check reference caused docutils problems with no ldesc.
The cygwinccompiler change produced a DeprecationWarning.

Cygwin separates import libraries from dynamic libraries (needed link time vs run time).

Let's see if I got the syntax right.

Next step will be hooking the temporary /tmp/libxx_z.so file handling into pytest's tempfile handling.

The old test seemed to pick up the 32-bit library, not the 64-bit one.
The new test should pick up the 64-bit one consistently when relevant.

Still need to get the test compiling in the alternate location.  Did I add -L/tmp?

I think /usr/lib/libz.so links to /usr/lib/libz.so.10, which in turn links to /usr/lib/libz.so.10.5.1 (numbers may differ), with the last being the actual library.  Ensure that is the one I copy for linking.

Maybe this will work better?  I should look into whether ELF `so`s store their basename.

Not the best solution, but I don't know how to check rpath in a manner that doesn't crash.