diff -r 000000000000 -r 6474c204b198 config/rebuild_check.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/config/rebuild_check.py	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,44 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import os
+import errno
+
+def mtime(path):
+    try:
+        return os.stat(path).st_mtime
+    except OSError as e:
+        if e.errno == errno.ENOENT:
+            return -1
+        raise
+
+def rebuild_check(args):
+    target = args[0]
+    deps = args[1:]
+    t = mtime(target)
+    if t < 0:
+        print target
+        return
+
+    newer = []
+    removed = []
+    for dep in deps:
+        deptime = mtime(dep)
+        if deptime < 0:
+            removed.append(dep)
+        elif mtime(dep) > t:
+            newer.append(dep)
+
+    if newer and removed:
+        print 'Rebuilding %s because %s changed and %s was removed' % (target, ', '.join(newer), ', '.join(removed))
+    elif newer:
+        print 'Rebuilding %s because %s changed' % (target, ', '.join(newer))
+    elif removed:
+        print 'Rebuilding %s because %s was removed' % (target, ', '.join(removed))
+    else:
+        print 'Rebuilding %s for an unknown reason' % target
+
+if __name__ == '__main__':
+    import sys
+    rebuild_check(sys.argv[1:])