Use $(MAKE), not make

For the sake of systems where we want gmake.
diff --git a/Makefile b/Makefile
index 1730f6d..0e272c6 100644
--- a/Makefile
+++ b/Makefile
@@ -68,7 +68,7 @@
 # note: for coverage testing, build with:
 # make CFLAGS='--coverage -g3 -O0'
 covtest:
-	make check
+	$(MAKE) check
 	programs/test/selftest
 	( cd tests && ./compat.sh )
 	( cd tests && ./ssl-opt.sh )
diff --git a/programs/Makefile b/programs/Makefile
index f62e2c6..26aaa8b 100644
--- a/programs/Makefile
+++ b/programs/Makefile
@@ -81,7 +81,7 @@
 all: $(APPS)
 
 $(DEP):
-	make -C ../library
+	$(MAKE) -C ../library
 
 aes/aescrypt2$(EXEXT): aes/aescrypt2.c $(DEP)
 	echo   "  CC    aes/aescrypt2.c"
diff --git a/tests/Makefile b/tests/Makefile
index 731dcd9..dee7fdb 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -92,7 +92,7 @@
 all: $(APPS)
 
 $(DEP):
-	make -C ../library
+	$(MAKE) -C ../library
 
 test_suite_aes.ecb.c : suites/test_suite_aes.function suites/test_suite_aes.ecb.data scripts/generate_code.pl suites/helpers.function suites/main_test.function
 	echo   "  Gen   $@"