Index: uspace/lib/libc/Makefile.toolchain
===================================================================
--- uspace/lib/libc/Makefile.toolchain	(revision 80e3c69a1f583bdaac3b94d8530a57dcfd5af230)
+++ uspace/lib/libc/Makefile.toolchain	(revision a68f7375e3e4f83a6876853e429edc329d00e4e4)
@@ -30,5 +30,5 @@
 	-fexec-charset=UTF-8 -fwide-exec-charset=UTF-32 -finput-charset=UTF-8 \
 	-fno-builtin -Wall -Wextra -Wno-unused-parameter -Wmissing-prototypes \
-	-nostdlib -nostdinc -pipe -g
+	-Werror-implicit-function-declaration -nostdlib -nostdinc -pipe -g
 LFLAGS = -M -N $(SOFTINT_PREFIX)/libsoftint.a
 AFLAGS =