#2193 -DFOO on the SWIG command line now sets FOO to 1 for
consistency with C/C++ compiler preprocessors. Previously
SWIG set FOO to an empty value.
Existing invocations of SWIG with `-DFOO` where the empty value
matters can be updated to `-DFOO=` which should work with both
old and new releases of SWIG.
*** POTENTIAL INCOMPATIBILITY ***
For swig-4.1.0 change:
See https://github.com/swig/swig/issues/2193
This patch is backwards compatible with older versions of SWIG.