build: Don't install ad and ipa man pages unnecessarily
    
    Don't install sssd-ipa and sssd-ad man pages if corresponding providers
    weren't built (i.e. --without-samba was specified to configure).
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit 4b9c236008b35d2155b94635a236ce7770db4728)