Bug 454449. Teach client.mk to fail more gracefully if it can't locate autoconf. r=ted.mielczarek

client.mk

@@ -93,6 +93,10 @@ endif
 # MacOS X 10.4 sends "no autoconf*" errors to stdout, discard those via grep
 AUTOCONF ?= $(shell which autoconf-2.13 autoconf2.13 autoconf213 2>/dev/null | grep -v '^no autoconf' | head -1)
 
+ifeq (,$(strip $(AUTOCONF)))
+$(error Couldn't find autoconf 2.13)
+endif
+
 MKDIR := mkdir
 SH := /bin/sh
 ifndef MAKE