fetchsvn: Add ignoreExternals attribute.

There are some SVN repositories out there which don't have revision information
tied to externals. By using ignoreExternals, fetchsvn won't fetch these
externals anymore, so the fetch won't fail with a checksum mismatch, should
there be some changes in some of those external repositories.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/build-support/fetchsvn/builder.sh

@@ -22,6 +22,7 @@ fi;
 # server's certificate.  This is perfectly safe: we don't care
 # whether the server is being spoofed --- only the cryptographic
 # hash of the output matters.
-echo 'p' | svn export -r "$rev" "$url" $out
+echo 'p' | svn export ${ignoreExternals:+--ignore-externals} \
+    -r "$rev" "$url" "$out"
 
 stopNest

pkgs/build-support/fetchsvn/default.nix

@@ -1,5 +1,5 @@
 {stdenv, subversion, sshSupport ? false, openssh ? null}: 
-{url, rev ? "HEAD", md5 ? "", sha256 ? ""}:
+{url, rev ? "HEAD", md5 ? "", sha256 ? "", ignoreExternals ? false}:
 
 let
   repoName = with stdenv.lib;
@@ -29,7 +29,7 @@ stdenv.mkDerivation {
   outputHashMode = "recursive";
   outputHash = if sha256 == "" then md5 else sha256;
   
-  inherit url rev sshSupport openssh;
+  inherit url rev sshSupport openssh ignoreExternals;
 
   impureEnvVars = [
     # We borrow these environment variables from the caller to allow