Makefile.in: check for configure being up to date

Fixes #934

Add a rule that errors out with a message if configure.ac is newer than
configure. This should catch times where someone is building from the
repo without running bootstrap.

Signed-off-by: Chris Packham <judge.packham@gmail.com>

Makefile.in

@@ -44,7 +44,7 @@ __silent_rmdir = $(call __silent,RMDIR,$1)rm -rf $1
 # Remove any suffix rules
 .SUFFIXES:
 
-all: Makefile build
+all: configure Makefile build
 
 ###############################################################################
 # Configuration variables
@@ -104,6 +104,10 @@ Makefile: Makefile.in
 	@echo "$< changed: you must re-run './configure'"
 	@false
 
+configure: configure.ac
+	@echo "$< changed: you must re-run './bootstrap'"
+	@false
+
 # If installing with DESTDIR, check it's an absolute path
 ifneq ($(strip $(DESTDIR)),)
   ifneq ($(DESTDIR),$(abspath /$(DESTDIR)))