anyway, simplify logic a bit:
Add Python 2.5 to the default list, but also mark it as incompatible if
wip/python25 is not present. Move the Darwin handling after setting a
default value.
Provide a new variable _PYTHON_VERSIONS_ACCEPTED that is filtered by
PYTHON_VERSIONS_INCOMPATIBLE. This helps to avoid providing broken
dependencies when a version is not supported as PYPKGPREFIX wouldn't be
set in that case.
pbulk-index-item prints a number of variables used by the parallel bulk
build code during either the build, the report or the upload phase.
pbulk-index checks whether multiple versions of the current package
could be build (e.g. because multiple Python versions are supported) and
uses pbulk-index-item for each possible combination.
Thanks to David Laight for explaining the different between using :[#]
in the body of a make target and in a clause of an .if.
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