Also detect gmime 2.6

Also detect gmime version 2.6 (Michael Biebl)

Signed-off-by: Tzafrir Cohen (License #5035) <tzafrir.cohen@xorcom.com>
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Merged revisions 360087 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 360098 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@360137 65c4cc65-6c06-0410-ace0-fbb531ad65f3