mingw-w64 defines __forceinline (and therefore FORCEINLINE) as "extern inline attribute((always_inline,gnu_inline)). This means that COM inline wrappers specify multiple storage classes and hence cannot be compiled.
Wine defines FORCEINLINE simply as "inline" (and uses "static" everywhere), so this is a non-issue for Wine. However, since Wine and mingw-w64 share the source code of widl and of most IDL headers, this patch changes the definition for both projects.
There's no reason to force inlining here, especially since the wrappers need to be manually enabled, and we don't need to match PSDK semantics where these wrappers don't even exist.
In practice, use "inline" instead of "inline" for GNU C targets, to preserve compatibility with C89 in mingw-w64 headers.