Add translation statistics
    
    The Makefile in install/po has a new target "msg-stats" which
    prints out statistics concerning the current pot and po files.
    Here is an example:
    
    % make msg-stats
    ipa.pot has 133 messages
    id.po:   107/133  80.5%   13 po untranslated,   13 missing,   26 untranslated
    kn.po:     4/133   3.0%  116 po untranslated,   13 missing,  129 untranslated
    pl.po:   120/133  90.2%    0 po untranslated,   13 missing,   13 untranslated
    
    Also update configure.ac to search for msgcmp, awk & sed programs.