Refactor Makefile

These changes make rdo build on my rpi, probably because this specific
GCC version requires arguments in the order we now supply.

We also disable ASAN in the debug build (for now), as it seems to cause
a segfault on my rpi.

Makefile

@@ -1,8 +1,14 @@
+CFLAGS = -Wall -Wextra -Werror -Wl,-z,now -D_FORTIFY_SOURCE=2
+CFLAGS_RELEASE = ${CFLAGS} -O2 -s
+CFLAGS_DEBUG = ${CFLAGS} -O0 -g -fsanitize=leak,undefined
+LIBS = -lbsd -lcrypt
+CC = gcc
+
 all: rdo.c
-	gcc -D_FORTIFY_SOURCE=2 -O2 -s -Wl,-z,now -lcrypt -lbsd -Wall -Wextra -Werror rdo.c -o rdo
+	${CC} ${CFLAGS_RELEASE} rdo.c -o rdo ${LIBS}
 
 debug: rdo.c
-	gcc -O0 -g -Wl,-z,now -fsanitize=leak,address,undefined -lcrypt -lbsd -Wall -Wextra -Werror rdo.c -o rdo
+	${CC} ${CFLAGS_DEBUG} rdo.c -o rdo ${LIBS}
 
 install: rdo
 	cp rdo /usr/bin/rdo