Revive the globstar globbing patch

[23Skidoo]

#2522 added support for globstar globbing (**/*) to extra-source-files and related fields; however the patch had some issues and wasn't merged. We should revive and finally merge it for the next Cabal release.

[23Skidoo]

@hvr told me that GHC 8 final will likely be delayed until the end of March, so maybe we'll have time to get this in.