Enable compilation and functionality on darwin(osx)

Enable compilation and functionality on darwin(osx)

Currently, the iotivity project does not support compiling/
running on Apple Darwin platforms.  This patch enables
compilation and minimal sample functionality for darwin.

It accomplishes this by adding 'darwin' as a valid
target_os in all build-scripts.  glib2 compiling/linking
is also added for the darwin build, which was previously
missing and blocking building connectivity-abstraction.

Many positions in the code had #ifdef __linux__, which has
been switched to also recognize __APPLE__.

Finally, the logger time-functionality used linux-specific
clock_gettime, so this functionality has been broken up
into an osalGetTime function, with an implementation for
__APPLE__ included in this patch.

Change-Id: Iddc2462db44198be90a975536e0273f63c521ab6
Signed-off-by: Charlie Lenahan <charlie.lenahan@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/668
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Erich Keane <erich.keane@intel.com>