OK, there are two parts:

* enable `GLOB_BRACE` if available.  This is a trivial change, I suggest we make it regardless, it's very low-risk and users will expect it.
* rewrite `**` to the 6 possible combinations.  I've changed the PR to always do this, without depending on `GLOB_BRACE`.  If you think that's too much new code, then I can make a PR for just the `GLOB_BRACE` addition and let this wait.