CONFIGURE: Properly detect libpopt on el7
    
    libpopt added pkg-config file in 1.16 but there are still distributions
    which has older version of library (el6, el7). And new features from
    libpopt are not used anywhere. Configure should try to detect as much as
    possible and users should not use workarounds with explicitely enabled
    variables as parameters e.g.
       ./configure POPT_LIBS="-lpopt "
    
    Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>