Revert previous QFileDialog fix for Solaris

This reverts previous commit 6d47f5a168e350a6d9cbddb343ff9d75ffbd2c04,
that broke compilation of QFileDialog in solaris (Sun CC).

Solaris defaults to Draft version of getpwnam_r if _POSIX_C_SOURCE is not
defined or < 199506L, and _POSIX_VERSION is defined as 199506L even when
_POSIX_C_SOURCE is not used, so it cannot be used to detect correct
posix support.

Task-number: QTBUG-21451
Change-Id: Ibbf2b9de1dea74e7d8b6003d1a4e3686e5858525
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Milla Pohjanheimo <milla.pohjanheimo@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>