Unfortunately the changes of 26bf86322 (Use SWIG-specific for
non-overloaded synthesized functions too, 2021-11-09) did break some
existing code bases using SWIG as they hardcoded the old wrapper
function names.
So turn this off by default and add a global variable allowing to enable
this, which can be done for a specific language only. This is ugly but,
unfortunately, there is no way to use the Language object from the C
function Swig_MethodToFunction(), so the only alternative would be to
add another parameter to it, but it already has 6 of them, so it
wouldn't really be that much better.
See #2366, #2368, #2370.
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