Makefile making mysterious rm command

Makefile

FILENAMES	+= ft_isalpha.c ft_isdigit.c ft_isalnum.c ft_isascii.c ft_isprint.c
FILENAMES	+= ft_toupper.c ft_tolower.c
TESTS		+= char_is

# Remark: I have a file named test_char_is.c that contains a main()

FILENAMES	+= # ... more ...
TESTS		+= # ... more ...

NAME		= libft.a

CC		= gcc
CFLAGS	+= -Wall -Wextra -Werror
LDFLAGS	+= -Wall -Wextra -Werror

ifeq ($(DEBUG), 1)
	CFLAGS	+= -fsanitize=address
	LDFLAGS	+= -fsanitize=address
else
	#CFLAGS	+= -O3 -fomit-frame-pointer -DNDEBUG -flto
	#LDFLAGS	+= -O3 -fomit-frame-pointer -DNDEBUG -flto
endif

SRCS	= $(FILENAMES)
OBJS	= $(addprefix build/, $(FILENAMES:.c=.o))

TESTTARGETS	= $(addprefix test-, $(TESTS))
TESTBINS	= $(addprefix build/, $(TESTTARGETS))

.PHONY: all clean fclean re test $(TESTTARGETS)

all: $(NAME)

re: fclean all

clean:
	rm -rf build/
	rm -f $(TESTBINS)

fclean: clean
	rm -f $(NAME)

build:
	mkdir build/

$(NAME): $(OBJS)
	ar rcs $@ $^

build/%.o: %.c libft.h | build
	$(CC) $(CFLAGS) -c $< -o $@

test: $(TESTBINS)
	for bin in $(TESTBINS); do \
		echo $$bin ; \
		$$bin ; \
		echo ; \
	done

build/test-%: build/test_%.o libft.a | build
	$(CC) $(LDFLAGS) -o $@ $^