Don't let broken symlinks halt linking

Library/Homebrew/extend/pathname.rb

@@ -298,7 +298,7 @@ class Pathname
       # NOTE only system ln -s will create RELATIVE symlinks
       quiet_system 'ln', '-s', src.relative_path_from(self.dirname), self.basename
       if not $?.success?
-        if symlink? && exist? || symlink?
+        if symlink? && exist?
           raise <<-EOS.undent
             Could not symlink file: #{src.expand_path}
             Target #{self} already exists as a symlink to #{readlink}.
@@ -320,6 +320,9 @@ class Pathname
             To list all files that would be deleted:
               brew link --overwrite --dry-run formula_name
             EOS
+        elsif symlink?
+          unlink
+          make_relative_symlink(src)
         elsif !dirname.writable_real?
           raise <<-EOS.undent
             Could not symlink file: #{src.expand_path}

Library/Homebrew/test/test_keg.rb

@@ -60,6 +60,11 @@ class LinkTests < Test::Unit::TestCase
     end
   end
 
+  def test_link_ignores_broken_symlinks_at_target
+    ln_s "/some/nonexistent/path", HOMEBREW_PREFIX/"bin/helloworld"
+    shutup { @keg.link }
+  end
+
   def test_link_overwrite
     touch HOMEBREW_PREFIX/"bin/helloworld"
     @mode.overwrite = true