port:RIOT:build: Use gnu99 c standard
port:RIOT:build: Use gnu99 c standard

Change-Id: I484fdbf90eaabe4bb5a161c1f5b570213442c5cf
Signed-off-by: Flavio Ceolin <979b9165500b0741b9d0500e2efd74fc1547bff7@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/12853
Reviewed-by: Kishen Maloor <c62ddea3cd661a9268bff15f07fc2229cdc9ac02@intel.com>
Tested-by: Kishen Maloor <c62ddea3cd661a9268bff15f07fc2229cdc9ac02@intel.com>

iotivity/iotivity-constrained,iotivity/iotivity-constrained,iotivity/iotivity-constrained,iotivity/iotivity-constrained