Append to CFLAGS; don't overwrite them. Don't use -Werror.
--- Makefile.orig	2021-03-21 17:05:45.000000000 -0500
+++ Makefile	2021-03-23 06:20:03.000000000 -0500
@@ -1,4 +1,4 @@
-CFLAGS = -Wall -Wextra -Werror -mmacosx-version-min=10.9 -framework Foundation -framework IOBluetooth
+CFLAGS += -Wall -Wextra -mmacosx-version-min=10.9 -framework Foundation -framework IOBluetooth
 
 DESTDIR =
 prefix = /usr/local