Hi,

I just tried to run `pip install numpy` on my OS X 10.10.3 box, and it proceeds to download and compile the tarball from PyPI from source (very slow). I see, however, that pre-compiled OS X wheel files are available on PyPI for OS X 10.6 and later.

Checking the code, it looks like pip is picking up the platform tag through `distutils.util.get_platform()`, which returns 'macosx-10.5-x86_64' on this machine. At root, I think this comes from the MACOSX_DEPLOYMENT_TARGET=10.5 entry in the Makefile at `python3.5/config-3.5m/Makefile`. I know that this value is used by distutils compiling python extension modules -- presumably so that they can be distributed to any target machine with OS X >=10.5 -- so that's good. But is this the right thing for pip to be using when checking whether a binary wheel is compatible? I see it mentioned in PEP 425, so perhaps this was already hashed out on the list.

Best,
Robert