Add gn package (#3396)

* Add gn package

* Add build dependencies

* Modify install syntax

* Change source_url and its hash to robustify package's installation

* Correction of the compilation environment

* Correction of the installation step

packages/gn.rb

+require 'package'
+
+class Gn < Package
+  description 'GN is a meta-build system that generates build files for Ninja.'
+  homepage 'https://gn.googlesource.com/gn/'
+  version 'dca877f'
+  source_url 'file:///dev/null'
+  source_sha256 'e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855'
+  
+  binary_url ({
+  })
+  binary_sha256 ({
+  })
+
+  depends_on 'python27' => :build
+  depends_on 'meson' => :build
+
+  def self.build
+    system "git clone https://gn.googlesource.com/gn"
+    ENV['C_INCLUDE_PATH']="#{CREW_PREFIX}/include/"
+    ENV['CXX']="g++"
+    Dir.chdir ("gn") do
+      system "python build/gen.py"
+      system "ninja -C out"
+    end
+  end
+
+  def self.install
+    system "install -Dm755 gn/out/gn #{CREW_DEST_PREFIX}/bin/gn"
+  end
+
+  def self.postinstall
+  end
+end