gcc-8 adds a new warning controlled by -Wcast-function-type, which is
implied on by Rebirth's default options. This new warning rejects
attempts to reinterpret_cast between function pointer types. While this
might provide some value when the function pointer was derived by taking
the address of a properly declared function provided elsewhere in the
same program, it is wrong when the function pointer is returned by an
external library call, such as GetProcAddress, which always returns a
placeholder type instead of the actual type of the target function.
Switch to using a union with type-punning, which achieves the same
effect as the cast, but does not count as a cast, and therefore does not
provoke the warning.
References: <https://github.com/dxx-rebirth/dxx-rebirth/issues/388>
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