Ensure that linker version scripts (used for symbol export control) always exist.

  
Using wildcard matching in the Makefile is not adequate to determine whether
an export file should exist for a module or not, so instead we'll just
create one if the module needs one, or copy the default one if it does not.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@256428 65c4cc65-6c06-0410-ace0-fbb531ad65f3