Fixed implicit-function-declaration warning for strnlen function

Fixed implicit-function-declaration warning for strnlen function

The strnlen is a GNU extension and not a standard C function and
is not avalible unless the macro _DEFAULT_SOURCE or _GNU_SOURCE is
defined.

It looks like _GNU_SOURCE already has a precidence so it was added
to the build script.

Change-Id: Ied150e5b97a965c1b4b240d364e49669bdad4f11
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/11523
Reviewed-by: Larry Sachs <larry.j.sachs@intel.com>
Reviewed-by: David Antler <david.a.antler@intel.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Rick Bell <richard.s.bell@intel.com>