ENH: specialize name of dev package by interpreter

[mattip]

tested by moving Python.h by hand and then running setup.py build_ext

[charris]

So both Python and PyPy supply the header? Is it the same header?

[mattip]

The file name is the same, but the include path is different and the contents are different, as are the contenct of many of the include files. Distribution package managers handle setting up the path (and virtualenv does as well) so it Just Works(tm)
I feel this commit is a bit silly, most users should be able to s/python/pypy/ without prompting, but we had someone complain

[charris]

OK, bit cumbersome but harmless. Thanks @mattip .