po: provide custom make rules for po file management

po: provide custom make rules for po file management

Historically we have relied on autopoint/gettextize to install a
standard po/Makefile.in.in. There is very limited scope for customizing
this and it also causes a bunch of extra stuff to be pulled into
configure.ac which potentially clashes with gnulib. Writing make rules
for po file management is no more difficult than any other rules libvirt
has, so stop using autopoint/gettextize.

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