On a macOS hosts the system ranlib does not understand ELF
files so using the "ranlib" command causes errors during cross-compilations.

The simplest way to fix it is to pass the RANLIB parameter provided to setup.py through to the distutils compiler machinery. This is analogous to the way the C/C++ cross-compiler is configured.

This change (in a GitHub PR) was required to proceed with crosscompiling numpy. It should help with other packages too (if they use distutils and need ranlib).