Avoid multiple downloads of postgresql and openssl sources

This happened in rake gem:windows because the downloads are defined as a rake task for each platform.
So the download was appended to the download task for every platform.
Now the download block is processed only once.

The code is borrowed from here:
b3dcbf69f1/lib/ruby_installer/build/utils.rb (L104-L144)

Rakefile.cross

@@ -7,6 +7,7 @@ require 'rake/clean'
 require 'rake/extensiontask'
 require 'rake/extensioncompiler'
 require 'ostruct'
+require_relative 'rakelib/task_extension'
 
 MISCDIR = BASEDIR + 'misc'
 
@@ -20,6 +21,7 @@ end
 
 class CrossLibrary < OpenStruct
 	include Rake::DSL
+	prepend TaskExtension
 
 	def initialize(for_platform, openssl_config, toolchain)
 		super()

rakelib/task_extension.rb

@@ -0,0 +1,46 @@
+# This source code is borrowed from:
+# https://github.com/oneclick/rubyinstaller2/blob/b3dcbf69f131e44c78ea3a1c5e0041c223f266ce/lib/ruby_installer/build/utils.rb#L104-L144
+
+module TaskExtension
+  # Extend rake's file task to be defined only once and to check the expected file is indeed generated
+  #
+  # The same as #task, but for #file.
+  # In addition this file task raises an error, if the file that is expected to be generated is not present after the block was executed.
+  def file(name, *args, &block)
+    task_once(name, block) do
+      super(name, *args) do |ta|
+        block.call(ta).tap do
+          raise "file #{ta.name} is missing after task executed" unless File.exist?(ta.name)
+        end
+      end
+    end
+  end
+
+  # Extend rake's task definition to be defined only once, even if called several times
+  #
+  # This allows to define common tasks next to specific tasks.
+  # It is expected that any variation of the task's block is reflected in the task name or namespace.
+  # If the task name is identical, the task block is executed only once, even if the file task definition is executed twice.
+  def task(name, *args, &block)
+    task_once(name, block) do
+      super
+    end
+  end
+
+  private def task_once(name, block)
+    name = name.keys.first if name.is_a?(Hash)
+    if block &&
+        Rake::Task.task_defined?(name) &&
+        Rake::Task[name].instance_variable_get('@task_block_location') == block.source_location
+      # task is already defined for this target and the same block
+      # So skip double definition of the same action
+      Rake::Task[name]
+    elsif block
+      yield.tap do
+        Rake::Task[name].instance_variable_set('@task_block_location', block.source_location)
+      end
+    else
+      yield
+    end
+  end
+end