Ticket #462 - add test for include file mntent.h
    
    https://fedorahosted.org/389/ticket/462
    Reviewed by: rmeggins
    Fixed by: cgrzemba
    Branch: master
    Fix Description: Add mntent.h to AC_CHECK_HEADERS - do not include
    mntent.h unless HAVE_MNTENT_H is defined.
    Platforms tested: RHEL6 x86_64
    Flag Day: no
    Doc impact: no