how to check that TidyLib is new enough?

[Jack Bates]

I am working on the build system for mod_atom, an Apache module:
http://code.google.com/p/mod-atom/

This module builds against TidyLib, so I use AC_CHECK_HEADER() to check
for buffio.h, and checkout the TidyLib source code from SourceForge
using CVS if it is not found:
http://mod-atom.googlecode.com/svn/trunk/configure.ac

Unfortunately there is a problem building mod_atom on Mac OS X because
the version of TidyLib distributed by MacPorts is old and does not
define the type 'TidyAllocator', leading to the following compile error:

[...]
filter.c:57: error: syntax error before 'TidyAllocator'
[...]

What is the "correct" way to use autoconf to check that version of
TidyLib it finds is new enough?

Thanks! Jack

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