bpo-36353: fix -R option of build_ext for OSX

[tovrstra]

For users who would like a workaround for this bug, make the following modification to your setup.py (tested with Cython and setuptools, but should also work in other situations):

from Cython.Distutils.build_ext import new_build_ext as build_ext
# alternative:
# from distutils.command import build_ext

class my_build_ext(build_ext):
    """Workaround for rpath bug in distutils for OSX."""

    def finalize_options(self):
        super().finalize_options()
        # Special treatment of rpath in case of OSX, to work around python
        # distutils bug 36353. This constructs proper rpath arguments for clang.
        # See https://bugs.python.org/issue36353
        if sys.platform[:6] == "darwin":
            for path in self.rpath:
                for ext in self.extensions:
                    ext.extra_link_args.append("-Wl,-rpath," + path)
            self.rpath[:] = []

setup(
    cmdclass={'build_ext': my_build_ext}
    # ...
)

https://bugs.python.org/issue36353

[the-knights-who-say-ni]

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[iritkatriel]

Closing this PR following the closure of its b.p.o issue (https://bugs.python.org/issue36353)