Add static compilation rule to makefile

Makefile

@@ -1,5 +1,6 @@
 CFLAGS = -Wall -Wextra -Werror -Wl,-z,now
 CFLAGS_RELEASE = ${CFLAGS} -O2 -s -D_FORTIFY_SOURCE=2
+CFLAGS_STATIC = ${CFLAGS} --static -s
 CFLAGS_DEBUG = ${CFLAGS} -O0 -g -fsanitize=undefined
 LIBS = -lcrypt
 CC = gcc
@@ -7,6 +8,9 @@ CC = gcc
 all: rdo.c
 	${CC} ${CFLAGS_RELEASE} rdo.c -o rdo ${LIBS}
 
+static: rdo.c
+	${CC} ${CFLAGS_STATIC} rdo.c -o rdo ${LIBS}
+
 debug: rdo.c
 	${CC} ${CFLAGS_DEBUG} rdo.c -o rdo ${LIBS}