broken makefile

makefile

#!/usr/bin/make -f
cc = gcc
cc_standard = -std=c17
cc_optimization = -Ofast -march=native
cc_warnings = -Werror -Wall -Wextra -Wpedantic -Wshadow -Wconversion \
	-Wmissing-prototypes -Wstrict-prototypes -Wold-style-definition -g
cc_link = -lgmp
target_file?=main.c

build:
	${cc} ${cc_standard} ${cc_optimization} ${cc_warnings} ${target_file} -o \
		$@ ${cc_link}

.PHONY: clean

style:
	astyle -rv --style=linux --indent=force-tab=4 --delete-empty-lines \
	--break-closing-braces --max-code-length=80 --lineend=linux --ascii \
	"*.c"

clean:
	rm -rf main