pypy installations don't usuallyy ship with pkg-config files,
we thus need to replicate what their version of distutils does.
In addition, we also try our best to build against other
pythons that do not have pkg-config files.
What is actually defined here varies wildly on different python-versions
for different platforms.
On my python2.7 on Windows len(sysconfig.get_config_vars()) returns 17,
whereas in my Ubuntu that number is 517!
Hence it is useful to be able to check which keys are available, as
well as allowing specifying a default option.
Mathieu Duponchelle
Havard Graff
Jussi Pakkanen