diff -r b0ec211da98a scripts/Makefile.build
--- a/scripts/Makefile.build	Fri Oct 05 10:51:53 2007 +0100
+++ b/scripts/Makefile.build	Tue Oct 09 22:45:45 2007 -0400
@@ -71,6 +71,8 @@ ifeq ($(CONFIG_XEN),y)
 ifeq ($(CONFIG_XEN),y)
 $(objtree)/scripts/Makefile.xen: $(srctree)/scripts/Makefile.xen.awk $(srctree)/scripts/Makefile.build
 	@echo '  Updating $@'
+	$(if $(shell echo a | $(AWK) '{ print gensub(/a/, "AA", "g"); }'),\
+        ,$(error 'Your awk program does not define gensub.  Use gawk or another awk with gensub'))
 	@$(AWK) -f $< $(filter-out $<,$^) >$@
 
 xen-src-single-used-m	:= $(patsubst $(srctree)/%,%,$(wildcard $(addprefix $(srctree)/,$(single-used-m:.o=-xen.c))))