Index: boot/Makefile.build
===================================================================
--- boot/Makefile.build	(revision c9f3e7e90bf5ef1a7d8f76f8717c35a796152375)
+++ boot/Makefile.build	(revision 37e8c4a1c218e8d69819bd4f761fa694e0884dad)
@@ -37,5 +37,5 @@
 
 AFLAGS =
-LFLAGS = --fatal-warnings
+LFLAGS = --fatal-warnings --warn-common
 
 # FIXME: This condition is a workaround for issue #693.
@@ -46,5 +46,5 @@
 COMMON_CFLAGS = $(INCLUDES) -O$(OPTIMIZATION) -imacros $(CONFIG_HEADER) \
 	-ffreestanding -fno-builtin -nostdlib -nostdinc \
-	-fexec-charset=UTF-8 -finput-charset=UTF-8
+	-fexec-charset=UTF-8 -finput-charset=UTF-8 -fno-common
 
 GCC_CFLAGS = -Wall -Wextra -Wno-unused-parameter -Wmissing-prototypes \