Don't assume make is gmake

*** 12,24 ****
  # express or implied. See the License for the specific language governing
  # permissions and limitations under the License.
  #
  
  all:
!         make -C testlib
!         make -C unit
  
  include ../s2n.mk
  
  clean:
!         make -C testlib decruft
!         make -C unit decruft
--- 12,24 ----
  # express or implied. See the License for the specific language governing
  # permissions and limitations under the License.
  #
  
  all:
!         $(MAKE) -C testlib
!         $(MAKE) -C unit
  
  include ../s2n.mk
  
  clean:
!         $(MAKE) -C testlib decruft
!         $(MAKE) -C unit decruft