SWIGing a source file <name>.i silently overwrites <name>.c if it is

SWIGing a source file <name>.i silently overwrites <name>.c if it is
present - at least the swigged file should be named <name>_wrap.c as
this is also SWIG's default. (Even better would be to generate the
wrapped sources in a different location, but I'll leave this for
later).

Newer versions of SWIG don't accept the -dnone flag any more.

Since virtually nobody uses SWIG with distutils, this should do no
harm.

Suggested be Martin Bless on c.l.p.