autoconf: Add configure target to main Makefile

While at it fill git version information in configure.ac
configure target needs autoconf, of course.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/Makefile b/Makefile
index ae4c0f2..d8cc835 100644
--- a/Makefile
+++ b/Makefile
@@ -595,6 +595,13 @@
 	chmod +x $@+
 	mv $@+ $@
 
+configure: configure.ac
+	rm -f $@ $<+
+	sed -e 's/@@GIT_VERSION@@/$(GIT_VERSION)/g' \
+	    $< > $<+
+	autoconf -o $@ $<+
+	rm -f $<+
+
 # These can record GIT_VERSION
 git$X git.spec \
 	$(patsubst %.sh,%,$(SCRIPT_SH)) \