Revert previous QFileDialog fix for Solaris

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>