Partial fix for #887242 (link extensions with dynamic_lookup in stead

of hard linking against the framework).

If $MACOSX_DEPLOYMENT_TARGET is set, and >= 10.3, during configure we
setup extensions to link with dynamic lookup. We also record the
value in the Makefile.

Distutils checks whether a value for MACOSX_DEPLOYMENT_TARGET was
recorded in the Makefile, and if it was insists that the current
value matches.

This is only a partial fix because it only applies to 2.4, and the
"two python problem" exists with Python 2.3 shipped with MacOSX 10.3,
which we have no influence over.