Propagate MACOSX_DEPLOYMENT_TARGET to environment

Apple switched default C++ standard libraries (from gcc's libstdc++ to
clang's libc++), but some pre-packaged Python environments such as
Anaconda are built against the old C++ standard library. Luckily, we
don't have to actually detect which C++ standard library was used to
build the Python interpreter. We just have to propagate
`MACOSX_DEPLOYMENT_TARGET` from the configuration variables to the
environment.

This workaround fixes #151.