Patch for autogen.sh and configure.ac to support automake 1.13

Follow-up report to bug #992047

MacPorts has upgraded automake to 1.13 [1], which again causes current trunk to fail to properly configure.

Attached (minimal) patch adds detection of automake 1.13 to 'autogen.sh', and replaces two obsolete macros [2] in 'configure.ac' to allow to configure and build trunk with automake 1.13 (on OS X).

*** Review needed with regard to 'AM_PROG_CC_STDC' ***
<http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/view/head:/configure.ac#L29>
Related notes in the release notes of automake 1.13 [2]:
> AM_PROG_CC_STDC: superseded by AC_PROG_CC since October 2002

The proposed superseding macro 'AC_PROG_CC' is already in use related to libcroco a few lines further down:
<http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/view/head:/configure.ac#L58>

One of the two can likely be removed (redundant?), but this ought to be reviewed by someone more familiar with autoconf (and Inkscape's copy of libcroco) than me.

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[1] <https://trac.macports.org/changeset/100840>
[2] <https://lists.gnu.org/archive/html/automake/2012-12/msg00038.html>