Conditional gem dependency installation within a gemspec (closes #2)

chef-encrypted-attributes.gemspec

@@ -36,10 +36,13 @@ Gem::Specification.new do |s|
   s.add_dependency 'chef', chef_version
 
   # Support old deprecated Ruby versions:
-  s.add_dependency 'mixlib-shellout', '< 1.6.1' if RUBY_VERSION < '1.9.3'
+  s.extensions << 'ext/mkrf_conf.rb'
+  if RUBY_VERSION < '1.9.3'
+    s.add_development_dependency 'mixlib-shellout', '< 1.6.1'
+  end
   if RUBY_VERSION < '2'
-    s.add_dependency 'highline', '< 1.7'
-    s.add_dependency 'ohai', '< 8'
+    s.add_development_dependency 'highline', '< 1.7'
+    s.add_development_dependency 'ohai', '< 8'
   end
 
   s.add_development_dependency 'rake', '~> 10.0'

ext/mkrf_conf.rb

@@ -0,0 +1,52 @@
+# encoding: UTF-8
+#
+# Author:: Xabier de Zuazo (<xabier@onddo.com>)
+# Copyright:: Copyright (c) 2015 Onddo Labs, SL. (www.onddo.com)
+# 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.
+#
+
+# Conditional gem dependency installation within a gemspec.
+#
+# Based on:
+# * http://www.programmersparadox.com/2012/05/21
+#     /gemspec-loading-dependent-gems-based-on-the-users-system/
+# * https://www.tiredpixel.com/2014/01/05
+#     /curses-conditional-ruby-gem-installation-within-a-gemspec/
+
+require 'rubygems/dependency_installer'
+
+di = Gem::DependencyInstaller.new
+
+begin
+  if RUBY_VERSION < '1.9.3'
+    puts "Installing mixlib-shellout < 1.6.1 because Ruby #{RUBY_VERSION}"
+    di.install 'mixlib-shellout', '< 1.6.1'
+  end
+  if RUBY_VERSION < '2'
+    puts "Installing highline < 1.7 because Ruby #{RUBY_VERSION}"
+    s.add_dependency 'highline', '< 1.7'
+    puts "Installing ohai < 8 because Ruby #{RUBY_VERSION}"
+    s.add_dependency 'ohai', '< 8'
+  end
+rescue => e
+  warn "#{$PROGRAM_NAME}: #{e}"
+  exit!
+end
+
+puts 'Writing fake Rakefile'
+# Write fake Rakefile for rake since Makefile isn't used
+File.open(File.join(File.dirname(__FILE__), 'Rakefile'), 'w') do |f|
+  f.write("task :default\n")
+end