### Please describe the problem.

git-annex can't compile on FreeBSD; specifically, the build fails due to issues with System.Posix.Files.

### What steps will reproduce the problem?

1. git clone git://git-annex.branchable.com/ git-annex
2. cd git-annex
3. stack setup
4. stack install

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

git-annex HEAD.

FreeBSD 11.1-RELEASE r321309 GENERIC amd64

### Please provide any additional information below.

Let me know if you'd like me to set up a FreeBSD development environment for you to SSH into - happy to do that if it helps you in any way.

Compilation failure is as follows:

    Configuring git-annex-6.20180807...
    git-annex-6.20180807: build (exe)
    Preprocessing executable 'git-annex' for git-annex-6.20180807...
    [124 of 586] Compiling Utility.DirWatcher.Kqueue ( Utility/DirWatcher/Kqueue.hs, .stack-work/dist/x86_64-freebsd/Cabal-1.24.2.0/build/git-annex/git-annex-tmp/Utility/Di
    rWatcher/Kqueue.o )

    /usr/home/duncan/code/git-annex/Utility/DirWatcher/Kqueue.hs:112:49: error:
        Not in scope: ‘Files.openFd’
        Module ‘System.Posix.Files’ does not export ‘openFd’.

    /usr/home/duncan/code/git-annex/Utility/DirWatcher/Kqueue.hs:112:66: error:
        Not in scope: data constructor ‘Files.ReadOnly’
        Module ‘System.Posix.Files’ does not export ‘ReadOnly’.

    /usr/home/duncan/code/git-annex/Utility/DirWatcher/Kqueue.hs:112:89: error:
        Not in scope: ‘Files.defaultFileFlags’
        Module ‘System.Posix.Files’ does not export ‘defaultFileFlags’.

    /usr/home/duncan/code/git-annex/Utility/DirWatcher/Kqueue.hs:132:15: error:
        Not in scope: ‘Files.closeFd’
        Module ‘System.Posix.Files’ does not export ‘closeFd’.

    /usr/home/duncan/code/git-annex/Utility/DirWatcher/Kqueue.hs:170:14: error:
        Not in scope: ‘Files.closeFd’
        Module ‘System.Posix.Files’ does not export ‘closeFd’.

    --  While building custom Setup.hs for package git-annex-6.20180807 using:
          /usr/home/duncan/code/git-annex/.stack-work/dist/x86_64-freebsd/Cabal-1.24.2.0/setup/setup --builddir=.stack-work/dist/x86_64-freebsd/Cabal-1.24.2.0 build exe:git
    -annex --ghc-options " -ddump-hi -ddump-to-file"
        Process exited with code: ExitFailure 1

> [[done]] --[[Joey]]
