Build subdirs with the same make(1) flavor

$ gmake
make -C sample
make[1]: "sample/Makefile" line 6: Missing dependency operator
make[1]: "sample/Makefile" line 18: Need an operator
make[1]: "sample/Makefile" line 23: Need an operator
make[1]: "sample/Makefile" line 25: Missing dependency operator
make[1]: "sample/Makefile" line 27: Missing dependency operator
make[1]: "sample/Makefile" line 29: Need an operator
make[1]: "sample/Makefile" line 30: Need an operator
make[1]: "sample/Makefile" line 32: Missing dependency operator
make[1]: "sample/Makefile" line 34: Need an operator
make[1]: Fatal errors encountered -- cannot continue
make[1]: stopped in sample
gmake: *** [Makefile:5: all] Error 1
diff --git a/Makefile b/Makefile
index 13012ad..ba5a624 100644
--- a/Makefile
+++ b/Makefile
@@ -2,10 +2,10 @@
 INSTALL_DIR=$(PREFIX)/include/xbyak
 
 all:
-	make -C sample
+	$(MAKE) -C sample
 
 clean:
-	make -C sample clean
+	$(MAKE) -C sample clean
 
 install:
 	mkdir -p $(INSTALL_DIR)