Let fetchurl retry failed downloads. Add NIX_CURL_FLAGS impure var

svn path=/nixpkgs/trunk/; revision=22006

pkgs/build-support/fetchurl/builder.sh

@@ -13,9 +13,11 @@ fi
 # cryptographic hash of the output anyway). 
 curl="curl \
  --location --max-redirs 20 \
+ --retry 3
  --disable-epsv \
  --cookie-jar cookies \
- --insecure"
+ --insecure \
+ $NIX_CURL_OPTIONS"
 
 
 tryDownload() {

pkgs/build-support/fetchurl/default.nix

@@ -91,6 +91,9 @@ stdenv.mkDerivation {
     # by definition pure.
     "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
 
+    # This variable allows the user to pass additional options to curl
+    "NIX_CURL_FLAGS"
+
     # This variable allows the user to override hashedMirrors from the
     # command-line.
     "NIX_HASHED_MIRRORS"