After the patch adding support for python 3 to the code generator,
python-six is required when building from git. Since I got 2 different
reports of SPICE build failures right after introducing it, it's
probably better to check for the needed python modules from configure,
and exit with an error if they are missing.
This commit adds a --enable-python-checks configure flag for that
though, since we only want to do that when building from git. It assumes
that people running from git will be running autogen.sh, while people
building from tarballs will run configure.
