Solaris 2.3 and 2.4 took their getpwnam_r() signature from draft 6
of the POSIX threads standard. Since Solaris 2.5 (1995), defining
_POSIX_PTHREAD_SEMANTICS opts-in to the non-draft version of
getpwnam_r(), and since Solaris 11.4 (2018), the non-draft version is
the default.

We already use AC_USE_SYSTEM_EXTENSIONS, which defines
_POSIX_PTHREAD_SEMANTICS, among other useful macros.

Thanks to Alan Coopersmith for assistance with Solaris history.

Signed-off-by: Simon McVittie <smcv@collabora.com>