Question

Why doesn’t Make link to foo.o?

$ ls
foo.c  foo_test.c  Makefile

$ cat Makefile 
.PHONY: test
test: foo_test
%_test: %_test.o foo.o

$ make
cc     foo_test.c   -o foo_test

Answer 1

Pattern rules MUST have a recipe associated with them. Any pattern rule without a recipe tells GNU make to delete that pattern rule. So, your line:

%_test: %_test.o foo.o

does nothing except delete the non-existent pattern rule to build %_test from %_test.o. You need to create a recipe if you want it to take effect:

%_test: %_test.o foo.o
        $(CC) -o $@ $(LIBS) $^

or whatever. However, this is completely not necessary for your example. You don’t need any rule at all for that, just write:

foo_test: foo_test.o foo.o

and let make’s built-in rules handle it.