When using a Python that has not been installed to build 3rd-party

When using a Python that has not been installed to build 3rd-party
modules, distutils does not understand that the build version of the
source tree is needed.

This patch fixes distutils.sysconfig to understand that the running
Python is part of the build tree and needs to use the appropriate
"shape" of the tree. This does not assume anything about the current
directory, so can be used to build 3rd-party modules using Python's
build tree as well.

This is useful since it allows us to use a non-installed debug-mode
Python with 3rd-party modules for testing. It as the side-effect that
set_python_build() is no longer needed (the hack which was added to
allow distutils to be used to build the "standard" extension modules).

This closes SF patch #547734.