Incorrect extension of PYTHONPATH

[Tristan Seligmann]

If the PYTHONPATH environment variable is already empty, doing something like:

PYTHONPATH="/path/to/dir:$PYTHONPATH" python [...]

results in a PYTHONPATH of "/path/to/dir:". As the empty segment at the end is interpreted as . (the current directory), this is undesireable. The easiest way to avoid this is to do:

PYTHONPATH="/path/to/dir${PYTHONPATH:+:$PYTHONPATH}" python [...]

This defect is present in axiom.test.test_xatop in Axiom, as well as in epsilon.release in Epsilon.