sal: unx/mutex.c: try to avoid system declarations with _GNU_SOURCE

Apparently the __USE_UNIX98 that is required to get the goodies is not
defined for some reason; apply some brute force to get it defined
(we don't care about anything older than RHEL5's glibc 2.5 anyway).)

For mysterious reasons "system.h" defines __USE_GNU but not _GNU_SOURCE,
which appears odd...

Change-Id: I97d75050a6be7f393d5cdb686f1357da052109f7
Reviewed-on: https://gerrit.libreoffice.org/10301
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>