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.
The thing is, you don't have to build it as a list. This is especially useful for the swig path, which never started as a list because it's a SCons.PathVariable.