Make the errors that happen when a user executes "make swig-py", but the Python bindings were not configured, a tiny bit less obscure.
* build/ac-macros/swig.m4 (SWIG_PY_COMPILE, SWIG_PY_LINK, SWIG_RB_COMPILE, SWIG_RB_LINK): Set variables which define commands to "none", rather than "", when the relevant bindings language is not configured.
Remove the configure option --enable-swig-bindings entirely. It did not actually enable anything. All it did was to conditionally skip a few configure tests, whilst its presence made the swig build system just that little bit more incomprehensible.
* build/ac-macros/swig.m4 (AC_ARG_ENABLE(swig-bindings)): Delete. (SWIG_BINDINGS_ENABLE): Delete. (SVN_FIND_SWIG): Do not test $svn_swig_bindings_enable_foo variables.