[yocto] How to combine BBCLASSEXTEND and variable export

[Hans Beckérus]

Hi. I have a recipe that use BBCLASSEXTEND to extend native. For the
normal/base version I wish to set/export an environment variable to be
picked up by configure.
But I do not wish this variable to be set by the native package. How
would I accomplish that? It was ok before when I had a package-native
recipe but since that is obsolete in favor for BBCLASSEXTEND I wish to
remove it. I think I covered all the other caveats like DEPEND and
PACKAGE/FILES. But this one is not as straight forward, or is it?

Thanks.
Hans