Index: uspace/lib/libc/Makefile.toolchain
===================================================================
--- uspace/lib/libc/Makefile.toolchain	(revision 94790d01466eefddc7dfc66d4537c2a02abef404)
+++ uspace/lib/libc/Makefile.toolchain	(revision 80ce111b0d6102444ae43ddf0684c88aa8b1f562)
@@ -31,5 +31,5 @@
 	-finput-charset=UTF-8 -ffreestanding -fno-builtin -nostdlib -nostdinc \
 	-Wall -Wextra -Wno-unused-parameter -Wmissing-prototypes \
-	-Werror-implicit-function-declaration -pipe -g -D__$(ENDIANESS)__
+	-Werror-implicit-function-declaration -Werror -pipe -g -D__$(ENDIANESS)__
 
 ICC_CFLAGS = -I$(LIBC_PREFIX)/include -O3 -imacros $(LIBC_PREFIX)/../../../config.h \
@@ -37,5 +37,5 @@
 	-finput-charset=UTF-8 -ffreestanding -fno-builtin -nostdlib -nostdinc \
 	-Wall -Wextra -Wno-unused-parameter -Wmissing-prototypes \
-	-Werror-implicit-function-declaration -pipe -g -D__$(ENDIANESS)__
+	-Werror-implicit-function-declaration -Werror -pipe -g -D__$(ENDIANESS)__
 
 CLANG_CFLAGS = -I$(LIBC_PREFIX)/include -O3 -imacros $(LIBC_PREFIX)/../../../config.h \