Don't assume make is gmake

@@ -12,13 +12,13 @@
 # express or implied. See the License for the specific language governing
 # permissions and limitations under the License.
 #
 
 all:
-        make -C testlib
-        make -C unit
+        $(MAKE) -C testlib
+        $(MAKE) -C unit
 
 include ../s2n.mk
 
 clean:
-        make -C testlib decruft
-        make -C unit decruft
+        $(MAKE) -C testlib decruft
+        $(MAKE) -C unit decruft