### Please describe the problem.

git-annex uses a custom setup script but does not have a custom-setup stanza. This causes
git-annex to be unbuildable with `cabal new-build` and I'm told it's also unbuildable with
the old `cabal build` under certain circumstances.

### What steps will reproduce the problem?

Try building git-annex with `cabal new-build`.

### What version of git-annex are you using? On what operating system?

git-annex-6.20171214 on Debian GNU/Linux 9.

### Please provide any additional information below.

[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log

$ cabal new-build --constraint="any.git-annex -testsuite" 
Resolving dependencies...
In order, the following will be built (use -v for more details):
 - git-annex-6.20171214 (exe:git-annex) -testsuite -dbus -concurrentoutput (requires build)
 - dummypkg-0 (lib) (first run)
Configuring git-annex-6.20171214 (all, legacy fallback)...
cabal: Failed to build git-annex-6.20171214 (which is required by dummypkg-0).
The failure occurred during the configure step. The exception was:
dieVerbatim: user error (cabal: '/usr/bin/ghc' exited with an error:

/home/matthew/dummypkg/dist-newstyle/tmp/src-3285/git-annex-6.20171214/Utility/FileSize.hs:10:1:
error:
Failed to load interface for ‘System.PosixCompat.Files’
It is a member of the hidden package ‘unix-compat-0.5.0.1’.
Perhaps you need to add ‘unix-compat’ to the build-depends in your .cabal
file.
Use -v to see a list of the files searched for.
)

# End of transcript or log.
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

Yeah, it's amazing! I've been using the version from the Debian repos and then
wanted to try building the new version for youtube-dl support.

> Revisited it and seem to have managed to add custom-setup back. [[done]]
> --[[Joey]]
