diff --git a/Makefile b/Makefile
index e92aa5c..9b0caba 100644
--- a/Makefile
+++ b/Makefile
@@ -1,15 +1,15 @@
 TARGET=kwh-cost
 
-CFLAGS=--std=c2x -O3 -Wall -Werror -Wextra -pedantic
+DESTDIR:=
+PREFIX:=/usr/local
+
+CFLAGS=--std=c99 -O3 -Wall -Werror -Wextra -pedantic
 
 SRC_DIR=src
-OBJ_DIR=obj
 
 DESTDIR=/
 
-.PHONY: all ${TARGET} clean install uninstall
-
-all: ${TARGET}
+.PHONY: clean install uninstall test
 
 ${TARGET}: ${SRC_DIR}/main.c ${SRC_DIR}/main.h
 	${CC} ${CFLAGS} ${LDFLAGS} $< -o $@
@@ -18,12 +18,12 @@ clean:
 	rm ${TARGET}
 
 install:
-	install -Dm755 ${TARGET} ${DESTDIR}usr/bin/${TARGET}
-	install -Dm644 LICENSE ${DESTDIR}usr/share/licenses/${TARGET}/LICENSE
+	install -Dm755 ${TARGET} ${DESTDIR}${PREFIX}/bin/${TARGET}
+	install -Dm644 LICENSE ${DESTDIR}${PREFIX}/share/licenses/${TARGET}/LICENSE
 
 uninstall:
-	rm -f ${DESTDIR}usr/bin/${TARGET}
-	rm -rf ${DESTDIR}usr/share/licenses/${TARGET}
+	rm -f ${DESTDIR}${PREFIX}/bin/${TARGET}
+	rm -rf ${DESTDIR}${PREFIX}/share/licenses/${TARGET}
 
 test:
 	@echo -n "Does input \"3.68 120 30d\" gives 317.95? "