Allow parallel make (-j) to work properly.

After some back and forth with the reporter, we came up with the necessary changes.

(closes issue #16489)
Reported by: Chainsaw
Patches:
      asterisk-1.6.2.1-parallel-make-minimal.patch uploaded by Chainsaw (license 723)
Tested by: Chainsaw, qwell


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@241230 65c4cc65-6c06-0410-ace0-fbb531ad65f3