build-sys: disable login-utils if shadow.h or utmp.h is missing

build-sys: disable login-utils if shadow.h or utmp.h is missing

Actually we could have also used UL_REQUIRES_LINUX because our
utmp usage and the shadow.h header is unlikely to be portable.
However only requiring these headers may help others who are
curious what needs to be done to port something.

Note, we could easily make the utmp stuff more portable by using
utmpx which is POSIX standard and on LINUX (glibc) basically just
renaming work. See getutxent(3).

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>