Fixes DM-1673: Allow override for broken SWIG installations.

SWIG needs an override on the env var SWIG_LIB when it is used outside
of the directory in which it was built and installed. This patch allows
an override via qserv's custom.py mechanism. Includes minor simplifications
in SConscript for swig.
Merge pull request #19 from LSST/u/danielw/DM-1673

Fixes DM-1673: Allow override for broken SWIG installations.