Thanks for the report.  This is another case of the problem described in Issue9516, in particular msg130666 except now it is hg invoking another Python during the build process.  The sysconfig part of the patch for Issue9516 applied to the "build" Python 2.7 used by hg will avoid the problem.  Or, as you found, as a workaround, ensure that the MACOSX_DEPLOYMENT_TARGET of the "build" Python matches that of the Python being built.  It's also only a problem if the "build" Python is 2.7 or 3.2.

By the way, since you've asked about it before, MACOSX_DEPLOYMENT_TARGET is a standard feature of the Apple gcc tool chain and is used to support builds for multiple versions.  See -mmacosx-version-min in the OS X man (1) gcc and http://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html