I've bisected it back to:

b07ae05a3dccef620c716e821c647413b951051 is the first bad commit
commit ab07ae05a3dccef620c716e821c647413b951051
Author: Matt Turner <mattst88@gmail.com>
Date:   Wed Sep 5 11:12:29 2012 -0700

    targets/dri-r600: Convert to automake

    v2: Andreas Boll <andreas.boll.dev@gmail.com>
        - Provide compatibility with scripts for the old Mesa build system

:100644 100644 9c9791b5137525bb22de3439806d565411a698aa
bb287d776c71a849f627af941bdbe0d514282ea1 M      configure.ac
:040000 040000 097ecdc98b02d01f522d90cae259f846fac14d64
c3659daefaf1f71454c058590a87603c250d4458 M      src

But I don't think that's much of a surprise and might be a bit of a red herring 

I've got a feeling the -m32 probably isn't being passed somewhere