build-aux: rewrite header ifdef checker in Python

build-aux: rewrite header ifdef checker in Python

As part of a goal to eliminate Perl from libvirt build tools,
rewrite the header-ifdef.pl tool in Python.

This was a straight conversion, manually going line-by-line to
change the syntax from Perl to Python. Thus the overall structure
of the file and approach is the same.

Tested-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>