Makefile: Correct recursive usage

Using $(MAKE) instead of "make", is special for gnu make. It
correctly processes Makeflags and parallel execution.

Makefile

@@ -14,14 +14,14 @@ all: bsmap
 %.o:%.cpp
 	$(CXX) $(CXXFLAGS) -c $< -o $@
 bsmap: $(OBJS1)
-	(cd samtools; make)
-	(cd gzstream; make)
+	$(MAKE) -C samtools
+	$(MAKE) -C gzstream
 	$(CXX) $(CXXFLAGS) $^ -o $@ $(THREAD) -lbam -lz -lgzstream
 
 clean:
 	rm -f *.o *~ bsmap
-	(cd samtools; make clean)
-	(cd gzstream; make clean)
+	$(MAKE) -C samtools clean
+	$(MAKE) -C gzstream clean
 install:
 	install -d $(BIN)
 	install ./bsmap $(BIN)