Automatically detect C99 flags for supported compilers.

Automatically detect C99 flags for supported compilers.

This patch set allows the required C99 command line flags to be
automatically set earlier in the build process.  Currently only gcc is
supported, but adding support for other compilers is straightforward.

Hard-coded '-std=c99' and '-std=gnu99' CFLAGS found throughout
iotivity SConscript files have been removed.  Feature test macros have
been added where needed to enable extensions that were disabled by
automatic addition of '-std=c99' to the CFLAGS build variable.

Change-Id: I6d05313aefff1911d1aaa2097954a1878874e73a
Signed-off-by: Ossama Othman <ossama.othman@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/495
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-by: Erich Keane <erich.keane@intel.com>