Made a proper Makefile as it should be fixing many errors, like ignoring VAR overwriting. Also, wherever meaningful, = was replaced with := that evaluates once. Also made use of so-called "Order-only prerequisites" in an install target to build a program if it is not already built. Also removed a dangerous rm -rf with rm -f for a LICENSE file and rmdir for a directory.

Makefile

@@ -1,9 +1,10 @@
-TARGET=kwh-cost
+TARGET := kwh-cost
 
-DESTDIR:=
-PREFIX:=/usr/local
+DESTDIR ?=
+PREFIX ?= /usr/local
 
-CFLAGS=--std=c99 -O3 -Wall -Werror -Wextra -pedantic
+CC ?= cc
+CFLAGS := --std=c99 -O3 -Wall -Werror -Wextra -pedantic
 
 
 .PHONY: clean install uninstall test
@@ -12,15 +13,16 @@ ${TARGET}: ${TARGET}.c
 	${CC} ${CFLAGS} ${LDFLAGS} $< -o $@
 
 clean:
-	rm ${TARGET}
+	rm -f ${TARGET}
 
-install:
+install: | ${TARGET}
 	install -Dm755 ${TARGET} ${DESTDIR}${PREFIX}/bin/${TARGET}
 	install -Dm644 LICENSE ${DESTDIR}${PREFIX}/share/licenses/${TARGET}/LICENSE
 
 uninstall:
 	rm -f ${DESTDIR}${PREFIX}/bin/${TARGET}
-	rm -rf ${DESTDIR}${PREFIX}/share/licenses/${TARGET}
+	rm -f ${DESTDIR}${PREFIX}/share/licenses/${TARGET}/LICENSE
+	rmdir ${DESTDIR}${PREFIX}/share/licenses/${TARGET}
 
 test:
 	@echo -n "Does input \"3.68 120 30d\" gives 317.95? "