That should fix the build on Ubuntu; also, it should add a clean target

common.mk

@@ -11,4 +11,7 @@ CFLAGS += -DINCLUDE_TESTS
 	ar crv $@ $^
 
 %.o: %.c
-	$(CC) $(CFLAGS) -c -o $@ $<
\ No newline at end of file
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+clean:
+	-rm -f $(OUTPUTS)

lib/Makefile

 include ../common.mk
 
+OUTPUTS := \
+	allocator.o
+
 all: allocator.o
 

src/Makefile

 -include ../common.mk
 
+OUTPUTS := bitreader.o \
+	   libhammer.a \
+	   test_suite
+
 all: libhammer.a test_suite
 
 test_suite: test_suite.o libhammer.a
-	$(CC) $(LDFLAGS) -o $@ $^
+	$(CC) -o $@ $^ $(LDFLAGS)
 
 libhammer.a: bitreader.o