[Cpp] Don't define ANTLR4CPP_EXPORTS in *consumers* of ANTLR runtime.

[BillyONeal]

This was originally submitted to vcpkg as https://github.com/microsoft/vcpkg/pull/29384

As currently written this is causing consumers of ANTLR's C++ runtime to define ANTLR4CPP_EXPORTS which means they are trying to __declspec(dllexport) rather than __declspec(dllimport) them.

See a repro in https://github.com/microsoft/vcpkg/issues/29326

Signed-off-by: Billy Robert O'Neal III [email protected]

[BillyONeal]

(The original author of this change is @TheReclif who deserves credit for authorship, not me)

[mcmikecreations]

Any progress on this? @BillyONeal

[BillyONeal]

Any progress on this? @BillyONeal

I'm not aware of additional changes necessary here?

[BillyONeal]

Any progress on this? @BillyONeal

I see it didn't merge cleanly so I pushed a merge.