Fix possible segv in a call to _assuan_debug.

* src/context.c (assuan_set_error): Do not pass NULL for %s in the
trace function.
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This likely fixes
GnuPG-bug-id: 1659
Update copyright info

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Update build system

* autogen.sh: Add --find-version stuff.  Taken from GnuPG.
* configure.ac: Change accordingly
Release 2.1.2.

* configure.ac: Set LT version to C4/A4/R2.
Post release updates

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