buildsystem: Don't force main to depend on everything else.

Directory 'main' only needs to depend on embedded modules. If no
module embedding is selected, the dependency is dropped.

Review: https://reviewboard.asterisk.org/r/3212/
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Merged revisions 408083 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 408084 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@408085 f38db490-d61c-443f-a65b-d21fe96a405b