Don't unnecessarily rebuild things on every run of 'make'.

Review: https://reviewboard.asterisk.org/r/2449/
........

Merged revisions 385745 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@385768 f38db490-d61c-443f-a65b-d21fe96a405b