Update configure script to be compatible with ptlib 2.10.9

With ptlib 2.10.9, the configure script fails due to grep returning multiple
matches for the pattern it searches for. This patch updates the pattern
matching to return only the actual version for the symbol searched for,
PTLIB_VERSION.

(closes issue ASTERISK-20980)
Reported by: Stefan Reuter
patches:
  ASTERISK-20980-1.patch uploaded by Stefan Reuter (license 5339)
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Merged revisions 380297 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 380298 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@380299 f38db490-d61c-443f-a65b-d21fe96a405b