The warning would appear when using the `ENUM_OPERATORS()` macro inside
of an `extern "C"` block.
Didn't cause a warning in master currently, but in the
`asset-browser-poselib` branch.
After macro expansion, there would be C++ code in code with C linkage
(`extern "C"`). So make sure the expanded C++ code always uses C++
linkage.
The syntax used is totally C++ compliant: the C++ standard requires that
in such nested linkage specifications, the innermost one determines the
linking language (e.g. see
https://timsong-cpp.github.io/cppwp/n4659/dcl.link#4).
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