--- xen-4.0.1/Config.mk
+++ xen-4.0.1.mod/Config.mk
@@ -14,7 +14,7 @@
 
 # Tools to run on system hosting the build
 HOSTCC      = gcc
-HOSTCFLAGS  = -Wall -Werror -Wstrict-prototypes -O2 -fomit-frame-pointer
+HOSTCFLAGS  = -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
 HOSTCFLAGS += -fno-strict-aliasing
 
 DISTDIR     ?= $(XEN_ROOT)/dist
--- xen-4.0.1/extras/mini-os/minios.mk
+++ xen-4.0.1.mod/extras/mini-os/minios.mk
@@ -6,7 +6,7 @@
 
 # Define some default flags.
 # NB. '-Wcast-qual' is nasty, so I omitted it.
-DEF_CFLAGS += -fno-builtin -Wall -Werror -Wredundant-decls -Wno-format -Wno-redundant-decls
+DEF_CFLAGS += -fno-builtin -Wall -Wredundant-decls -Wno-format -Wno-redundant-decls
 DEF_CFLAGS += $(call cc-option,$(CC),-fno-stack-protector,)
 DEF_CFLAGS += $(call cc-option,$(CC),-fgnu89-inline)
 DEF_CFLAGS += -Wstrict-prototypes -Wnested-externs -Wpointer-arith -Winline
--- xen-4.0.1/tools/libxc/Makefile
+++ xen-4.0.1.mod/tools/libxc/Makefile
@@ -57,7 +57,7 @@
 
 -include $(XEN_TARGET_ARCH)/Makefile
 
-CFLAGS   += -Werror -Wmissing-prototypes
+CFLAGS   += -Wmissing-prototypes
 CFLAGS   += $(INCLUDES) -I. -I../xenstore -I../include
 
 # Needed for posix_fadvise64() in xc_linux.c