Make alltests depend on libdiffpy.

This ensures that unit tests are linked with up to date shared library.
Do not shorten extended rcalc unless the difference is sizable.

This makes it more likely that optimized evaulation works after small
changes in displacement parameters.  The extended r-grid is shortened
only for sizable differences - this prevents updates to optimum qstep
which would trigger a full calculation of the Debye sum.
Ignore sconsvars.py file for local build preferences.