man/: Install suauth.5 only if feature exists

man/: Install suauth.5 only if feature exists

Support for /etc/suauth only exists if su is installed without
PAM support. If su is not installed (--without-su) or if PAM
support is enabled (default), do not install suauth.5 manual
page.

The SU_ACCESS preprocessor definition is used to decide if
feature exists or not. See links for more details.

Link: https://github.com/shadow-maint/shadow/blob/65723c3c4704fde4cc8c94299123d3eeca81aadf/src/su.c#L558-L581
Link: https://github.com/shadow-maint/shadow/blob/65723c3c4704fde4cc8c94299123d3eeca81aadf/configure.ac#L628-L654
Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>