libgo: update configure.ac to upstream GCC

The libgo/configure.ac file in upstream GCC was changed in

https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=6a6d3817afa02bbcd2388c8e005da6faf88932f1

This adds an automake conditional that is not used, but will be used
in the future when and if libgo is ported to Darwin.

This CL makes the corresponding change in this repo.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/546435