btrfs: fix build to link against libgcc_s so that pthread_cancel is linked

Currently, the command btrfs scrub start <filesystem> attempts to invoke the pthread_cancel
routine and ends with an error melibgcc_sssage about it being missing. This prevents btrfs
scrub status from maintaining proper statistics about the running scrub.

Close #1127.

pkgs/tools/filesystems/btrfsprogs/default.nix

@@ -13,6 +13,9 @@ stdenv.mkDerivation {
 
   buildInputs = [ zlib libuuid acl attr e2fsprogs lzo ];
 
+  # for btrfs to get the rpath to libgcc_s, needed for pthread_cancel to work
+  NIX_CFLAGS_LINK = "-lgcc_s";
+
   postPatch = ''
     cp ${./btrfs-set-received-uuid.c} btrfs-set-received-uuid.c
   '';