kvm tools: Add WERROR to Makefile for disabling -Werror

This patch adds a WERROR variable to Makefile that can be used as follows:

  make WERROR=0

to disable '-Werror' while building. This is useful while doing 'git bisect',
for example.

Suggested-by: Ingo Molnar <mingo@elte.hu>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

Makefile

@@ -1,3 +1,7 @@
+#
+# Define WERROR=0 to disable -Werror.
+#
+
 ifeq ($(strip $(V)),)
 	E = @echo
 	Q = @
@@ -111,7 +115,10 @@ DEFINES	+= -D_GNU_SOURCE
 KVM_INCLUDE := include
 CFLAGS	+= $(CPPFLAGS) $(DEFINES) -I$(KVM_INCLUDE) -I../../include -I../../arch/$(ARCH)/include/ -Os -g
 
-WARNINGS += -Werror
+ifneq ($(WERROR),0)
+	WARNINGS += -Werror
+endif
+
 WARNINGS += -Wall
 WARNINGS += -Wcast-align
 WARNINGS += -Wformat=2