The API docs extractor, ESX code generator and keycodemapdb tools
rely on python. Historically every platform that this present, but
with switch to Python3 by default, we're increasingly seeing
installs without a /usr/bin/python.

This tightens up the check during configure, so it exits immediately
if python is missing, rather than leaving an empty $(PYTHON) make
variable which leads to more obscure errors later.

Also add it as a build dep for Mingw, since Fedora build roots no
longer get python2 by default. This was not previously a major
problem, since both ESX & API generated files were included in
EXTRA_DIST, but the keycodemapdb generated files are not, so we
require python all the time now.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>