build-sys: check for POSIX utmpx usage

build-sys: check for POSIX utmpx usage

We simply check for utmpxname and updwtmpx functions to disable
all programs which are not POSIX utmpx compatible, even though
last(1) and utmpdump(1) does not use them, see below.

utmpx.h is used in:
  login agetty write lslogins last runuser su utmpdump wall

Non-Posix utmpx usage:
   utmpxname        -> login agetty write lslogins
   updwtmpx         -> login agetty                     runuser su
   _PATH_.TMP       -> login agetty write lslogins last runuser su
   utmpx.ut_addr_v6 -> login                       last            utmpdump

POSIX utmpx usage:
   wall

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