I have found this useful. It's caught a few errors.
Everything currently compiles (in my limited testing), so it's a good way to
keep it that way.
It could use some more testing on exotic archs so as to not break builds.
Index: sbin/Makefile.inc
===================================================================
RCS file: /cvs/src/sbin/Makefile.inc,v
retrieving revision 1.6
diff -u -p -r1.6 Makefile.inc
--- sbin/Makefile.inc 11 Jul 2014 13:13:04 -0000 1.6
+++ sbin/Makefile.inc 6 Jan 2016 18:55:55 -0000
@@ -2,4 +2,4 @@
BINDIR?= /sbin
LDSTATIC= ${STATIC}
-COPTS+= -Werror-implicit-function-declaration
+COPTS+= -Werror-implicit-function-declaration -Wformat
-Werror=format
Index: bin/Makefile.inc
===================================================================
RCS file: /cvs/src/bin/Makefile.inc,v
retrieving revision 1.6
diff -u -p -r1.6 Makefile.inc
--- bin/Makefile.inc 11 Jul 2014 13:13:04 -0000 1.6
+++ bin/Makefile.inc 6 Jan 2016 18:55:07 -0000
@@ -2,4 +2,4 @@
BINDIR?= /bin
LDSTATIC= ${STATIC}
-COPTS+=-Werror-implicit-function-declaration
+COPTS+=-Werror-implicit-function-declaration -Wformat -Werror=format
