UseSWIG: Undocumented SWIG_MODULE_${name}_EXTRA_FLAGS no longer honored
It was reported in !2243 (closed) that the internal variable SWIG_MODULE_${name}_EXTRA_FLAGS
could be used by projects to set per-target flags, but this was regressed by CMake 3.12.0. Even though this is an undocumented internal variable we should still support such projects until they can convert to a first-class interface for doing this.