honor the MacOSX SDK when one is specified.

This is needed to be able to build using the 10.4u SDK while running
on OSX 10.6.

This is a fixed version of the patch in r80963, I've tested this patch
on OSX and Linux.

the OSX 10.4u SDK on MacOSX 10.6 by honoring the specified
SDK when looking for files.

to "sys.platform == 'mac'" and that is
dead code because it refers to a platform
that is no longer supported (and hasn't been
supported for several releases).

Fixes issue #7908 for the trunk.

removed the local copy of xxmodule, and skip only test_build_ext when xxmodule is not found, not the whole unittest

But when the "dl" module is available, it gives a py3k deprecation warning.

reverting partially distutils to its 2.6.x state so 2.7a4 looks more like the 2.7b1 in this. the whole revert will occur after a4 is tagged

with -OO.  Tests requiring docstrings are skipped.  Patch by
Brian Curtin, thanks to Matias Torchinsky for helping review and
improve the patch.

Fixed #7748: now upload and register commands don't need to force the encoding anymore : DistributionMetada returns utf8 strings