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> Beyond that, I don't understand why the patch behavior depends on 
> whether the srcdir is an absolute path or not.
The os.path.isabs() test is actually redundant since os.path.join(base, srcdir) == srcdir if srcdir is an absolute path. I only added that test to be explicit about what happens if the original value is an absolute path. 
> I often use absolute paths to configure a build. If not in a build 
> directory, 'srcdir' should always return either the proposed value 
> based on what get_makefile_filename() returns or it should return 
> None.
Removing the isabs() test would not change the calculated value as I indicated above.
Using None would be the sanest option, but I don't know if any current code depends on the value being a string.
> One other point: distutils still has its own copy of sysconfig. 
> Strong consideration should be given to making a similar change there.
For a source build, distutils.sysconfig.get_config_var('srcdir') gives the right answer as an absolute path, *except* when the current working directory contains sys.executable. I think it should always return an absolute path. The fact that currently it does not probably explains the "mysteriously" comment below from distutils/test/support.py:
def _get_xxmodule_path():
 srcdir = sysconfig.get_config_var('srcdir')
 candidates = [
 # use installed copy if available
 os.path.join(os.path.dirname(__file__), 'xxmodule.c'),
 # otherwise try using copy from build directory
 os.path.join(srcdir, 'Modules', 'xxmodule.c'),
 # srcdir mysteriously can be $srcdir/Lib/distutils/tests when
 # this file is run from its parent directory, so walk up the
 # tree to find the real srcdir
 os.path.join(srcdir, '..', '..', '..', 'Modules', 'xxmodule.c'),
 ]
 for path in candidates:
 if os.path.exists(path):
 return path
BTW, I was wrong in my earlier message when I claimed that srcdir == get_path('stdlib') for an installed python. That is only true if the relative srcdir is '..'. The attached patch removes that check from the unit tests.