Make git more attractive for managing work-in-progress.  Especially
convenient when a potential patch set needs to be tested on multiple
platforms since one can use git to keep all the test environments in sync
independent of a subversion server.

Now the Asterisk version will show the exact git SHA5 that was used when
building (still appended by "M" if there are local modifications) from a
git clone of the Asterisk repository so the developer can more easily know
what is actually under test.

You will now get this:

  $ asterisk -V
  Asterisk GIT-1698298

Instead of this:

  $ asterisk -V
  Asterisk UNKNOWN__and_probably_unsupported

This has zero impact for those not using git with the exception of an
extra test in the configure script to gather git's path.  This is
necessary to prevent "sudo make install" from failing since git may not be
in the path in make's shell environment.

(closes issue ASTERISK-20483)
Reported by: Shaun Ruffell
Patches:
      0001-build_tools-Allow-Asterisk-to-report-git-SHAs-in-ver.patch (license #5417) patch uploaded by Shaun Ruffell
      Modified
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Merged revisions 375189 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 375190 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 375191 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@375192 65c4cc65-6c06-0410-ace0-fbb531ad65f3