Fix crashes on other platforms caused by interference from Darwin weak symbol support.

Support weak symbols on a platform specific basis.  The Mac OS X (Darwin)
support must be isolated from the other platforms because it has caused
other platforms to crash.  Several other platforms including Linux have
GCC versions that define the weak attribute.  However, this attribute is
only setup for use in the code by Darwin.

(closes issue ASTERISK-18728)
Reported by: Ben Klang

Review: https://reviewboard.asterisk.org/r/1617/
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Merged revisions 348647 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 348648 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@348649 65c4cc65-6c06-0410-ace0-fbb531ad65f3