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.
Im' nitpicking but it seems CPPPATH is always build as a list in Qserv scons scripts, so this function may be useless? We may test if env["CPPPATH"] is a list using Scons.utils.is_list and raise and exception if not?