Makefile: how to specify library name at the end of command using patsubst

I am new to Makefile, C and Linux. I am using gcc & ubuntu. I encountered a problem while trying to compile the code with a link to a library.

Here is my problem. I have:

a_tests.c & b_tests.c files in "tests" folderlib.a file in "build" folder

Here is the codes in Makefile related to the problem:

CFLAGS=-g -O2 -Wall -Wextra -Isrc -DNDEBUG $(OPTFLAGS)TARGET=build/lib.aTEST_SRC=$(wildcard tests/*_tests.c)TESTS=$(patsubst %.c,%,$(TEST_SRC))tests: CFLAGS += $(TARGET)tests: $(TESTS)

When the tests files are compiled, "undefined reference to 'XXXfunction'"errors will be prompted. Because what executed behind is

gcc -g -O2 -Wall -Wextra -Isrc -DNDEBUG build/lib.a tests/a_tests.c -o test/a_testsgcc -g -O2 -Wall -Wextra -Isrc -DNDEBUG build/lib.a tests/b_tests.c -o test/b_tests

But "build/lib.a" should be placed after the output file name to solve it (If I manually type in the below commands, the codes would be successfully compiled), ie:

gcc -g -O2 -Wall -Wextra -Isrc -DNDEBUG tests/a_tests.c -o test/a_tests build/lib.a gcc -g -O2 -Wall -Wextra -Isrc -DNDEBUG tests/b_tests.c -o test/b_tests build/lib.a 

But I don't know how to change in the Makefile, I tried -l -L options, they didn't work. It would warn that "cannot find the .a file". Any help would be appreciated. Thank you so much in advance!