configure: check explicitely for python dev (include) files

configure: check explicitely for python dev (include) files

As discussed at [1] we have received reports that the implicit check
using cffi.FFI().verify() is not reliable in all environments. Since
we already use pkg-config, and the python dev package should include a
.pc file [2], add an extra check using pkg-config.  On at least
Debian, we have to know which version of python dev files with are
looking for, so calculate that first.

[1]: id:87im1g35ey.fsf@tethera.netid:87im1g35ey.fsf@tethera.net,
[2]: checked on Debian and Fedora