Add pandoc build.

config/patches/pandoc/static_build.patch

+--- pandoc.cabal.orig	2017-05-12 11:18:36.178595538 -0400
++++ pandoc.cabal	2017-05-12 11:24:38.438595538 -0400
+@@ -310,6 +310,7 @@
+      cpp-options:   -DHTTP_CLIENT
+   if flag(embed_data_files)
+      cpp-options:   -DEMBED_DATA_FILES
++     ld-options:  -static -pthread
+      Build-Tools:   hsb2hs >= 0.3.1
+      other-modules: Text.Pandoc.Data
+   if os(windows)

config/software/gitlab-rails.rb

@@ -43,7 +43,7 @@ dependency 'rsync'
 dependency 'libicu'
 dependency 'postgresql'
 dependency 'postgresql_new'
-dependency 'python-docutils'
+dependency 'pandoc'
 dependency 'krb5'
 dependency 'registry'
 dependency 'gitlab-pages'

config/software/pandoc.rb

+#
+## Copyright:: Copyright (c) 2017 GitLab Inc.
+## License:: Apache License, Version 2.0
+##
+## Licensed under the Apache License, Version 2.0 (the "License");
+## you may not use this file except in compliance with the License.
+## You may obtain a copy of the License at
+##
+## http://www.apache.org/licenses/LICENSE-2.0
+##
+## Unless required by applicable law or agreed to in writing, software
+## distributed under the License is distributed on an "AS IS" BASIS,
+## WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+## See the License for the specific language governing permissions and
+## limitations under the License.
+##
+#
+
+name 'pandoc'
+default_version '1.19.2.1'
+
+license 'COPYING.md'
+license 'COPYRIGHT'
+
+source url: "https://hackage.haskell.org/package/pandoc-#{version}/pandoc-#{version}.tar.gz",
+       sha256: '08692f3d77bf95bb9ba3407f7af26de7c23134e7efcdafad0bdaf9050e2c7801'
+
+relative_path "pandoc-#{version}"
+
+build do
+  env = with_standard_compiler_flags(with_embedded_path)
+
+  flags = [ "--prefix=#{install_dir}/embedded",
+            "-fembed_data_files",
+            "--disable-shared",
+            "--disable-executable-dynamic"
+          ].join(' ')
+
+  command 'cabal update'
+  command 'cabal install --only-dependencies'
+  command 'cabal install hsb2hs'
+
+  # Patch Pandoc cabal configuration file to include static flags.
+  patch source: 'static_build.patch', target: 'pandoc.cabal'
+
+  command "cabal configure #{flags}", env: env
+  command 'cabal build', env: env
+  command 'cabal copy'
+end