fix i18n build problem
    
    There was a typo in install/po/Makefile.in which caused (some) of
    the .po files to be overwritten because the test to see if a po
    file existed had a typo in it.
    
    This patch also removes the unnecessary rebuilding of the pot which was
    happening when using the "all" target (the default). The pot file now
    must be manually remade, which is what we want.
    
    Added a new target "mo-files" to manually generate the .mo files.
    This is useful to run before checking in a new .po file just to
    assure it "compiles" and we don't have to discover this during a
    build.