diff -uNr moreutils-0.63/is_utf8/Makefile moreutils-0.63.mod/is_utf8/Makefile
--- moreutils-0.63/is_utf8/Makefile	2019-01-09 17:14:34.000000000 +0200
+++ moreutils-0.63.mod/is_utf8/Makefile	2020-01-03 20:49:32.066770922 +0200
@@ -37,8 +37,8 @@
 SONAME = $(LINKERNAME).$(VERSION)
 REALNAME = $(SONAME).$(MINOR).$(RELEASE)
 
-CC = gcc
-CFLAGS = -O3 -Wextra -Wall -ansi -Wstrict-prototypes
+CC ?= gcc
+CFLAGS += -Wextra -Wall -ansi -Wstrict-prototypes
 
 $(NAME): $(OBJ)
 	$(CC) $(CFLAGS) -o $(NAME) $(OBJ)