Avoid "most vexing parse" warning in certain versions of NVCC

[colesbury]

No description provided.

[colesbury]

This one is a bit weird, but I'm seeing -Wvexing-parse warnings in some versions of nvcc. I'm not entirely sure what's happening, but I think that it's a combination of:

• The void from PyAPI_FUNC(const PyConfig*) _Py_GetConfig(void); is getting stripped out by nvcc's frontend
• Clang is then giving a "most vexing parse" warning

I couldn't figure out any way to avoid this other than moving the declaration out of the function.

[vstinner]

Merged, thanks.