ENH: Don't add system library directories to rpath

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.

Author: DWesl

distutils/unixccompiler.py

@@ -148,6 +148,15 @@ class UnixCCompiler(CCompiler):
         dylib_lib_extension = ".dll"
         dylib_lib_format = "cyg%s%s"
 
+    def _fix_lib_args(self, libraries, library_dirs, runtime_library_dirs):
+        """Remove standard library path from rpath"""
+        libraries, library_dirs, runtime_library_dirs = super()._fix_lib_args(
+            libraries, library_dirs, runtime_library_dirs)
+        libdir = sysconfig.get_config_var('LIBDIR')
+        if runtime_library_dirs and (libdir in runtime_library_dirs):
+            runtime_library_dirs.remove(libdir)
+        return libraries, library_dirs, runtime_library_dirs
+  
     def preprocess(
         self,
         source,