Makefile: update scripts when build-time parameters change

Currently, running:

  make SHELL_PATH=/bin/bash &&
  make SHELL_PATH=/bin/sh

will not rebuild any shell scripts in the second command,
leading to incorrect results when building from an unclean
working directory.

This patch introduces a new dependency meta-file to notice
the change.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/.gitignore b/.gitignore
index c60c5a3..6535cd7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,6 +3,7 @@
 /GIT-LDFLAGS
 /GIT-GUI-VARS
 /GIT-PREFIX
+/GIT-SCRIPT-DEFINES
 /GIT-USER-AGENT
 /GIT-VERSION-FILE
 /bin-wrappers/