po: provide custom make rules for po file management
    
    Historically we have relied on intltool to install a standard
    po/Makefile.in.in which has very limited scope for customization.
    intltool is deprecated in favour of standard gettextize tools,
    but these share the same disadvantages.
    
    Writing make rules for po file management is no more difficult
    than any other rules virt-viewer has, so stop using intltool
    and don't use gettextize ether.
    
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>