libtorrent: fails with clang

Closes Homebrew/homebrew#11252.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/libtorrent.rb

@@ -1,15 +1,25 @@
 require 'formula'
 
 class Libtorrent < Formula
-  url 'http://libtorrent.rakshasa.no/downloads/libtorrent-0.12.9.tar.gz'
   homepage 'http://libtorrent.rakshasa.no/'
+  url 'http://libtorrent.rakshasa.no/downloads/libtorrent-0.12.9.tar.gz'
   md5 'b128bbd324f03eb42ef5060080f87548'
 
   depends_on 'pkg-config' => :build
   depends_on 'libsigc++'
 
+  # Upstream says this is fixed in 0.13.x series, so check if this is
+  # still needed when the next stable libtorrent release is made.
+  fails_with :clang do
+    build 318
+  end
+
   def install
-    system "./configure", "--prefix=#{prefix}", "--disable-debug", "--disable-dependency-tracking", "--with-kqueue", "--enable-ipv6"
+    system "./configure", "--prefix=#{prefix}",
+                          "--disable-debug",
+                          "--disable-dependency-tracking",
+                          "--with-kqueue",
+                          "--enable-ipv6"
     system "make"
     system "make install"
   end