Description: Add allow-bsd flag for Debian GNU/kFreeBSD
 On Debian GNU/kFreeBSD (FreeBSD kernel plus glibc), the BSD flavour of
 sendfile is not usable (struct sf_hdtr is declared with -D_BSD_SOURCE=1,
 but has no definition), and the Linux flavour doesn't work either since
 MSG_MORE is absent.  However, the fallback version works fine.
 .
 Unfortunately, Cabal treats kfreebsdgnu as os(freebsd), and os(kfreebsdgnu)
 doesn't work.  So, to make it possible to select the fallback
 implementation on GNU/kFreeBSD, I believe a flag is necessary.
Author: Colin Watson <cjwatson@debian.org>
Forwarded: https://github.com/kazu-yamamoto/simple-sendfile/pull/13
Last-Update: 2013-06-14

Index: b/simple-sendfile.cabal
===================================================================
--- a/simple-sendfile.cabal
+++ b/simple-sendfile.cabal
@@ -12,6 +12,10 @@
 Cabal-Version:          >= 1.10
 Build-Type:             Simple

+Flag allow-bsd
+  Description:          Allow use of BSD sendfile (disable on GNU/kFreeBSD)
+  Default:              True
+
 Library
   Default-Language:     Haskell2010
   GHC-Options:          -Wall
@@ -21,7 +25,7 @@
                       , network
                       , bytestring
   -- NetBSD and OpenBSD don't have sendfile
-  if os(freebsd)
+  if os(freebsd) && flag(allow-bsd)
     CPP-Options:        -DOS_BSD
     Other-Modules:      Network.Sendfile.BSD
                         Network.Sendfile.IOVec