Fix curses for supported Ruby versions, including Ruby 1.9.3 .

d85ea78 installs curses, because curses has been removed from the
standard library in Ruby 2.1.0 onwards. However, the curses gem does
not appear to work with Ruby 1.9.3 . Thus, it cannot be added as a
dependency in `sidekiq-spy.gemspec`.

`Gemfile` supports the notion of platforms. However, `Gemfile` is not
used at build-time. Add curses as a conditional dependency at install-
time, using `ext/mkrf_conf.rb`, based on

    http://en.wikibooks.org/wiki/Ruby_Programming/RubyGems#How_to_install_different_versions_of_gems_depending_on_which_version_of_ruby_the_installee_is_using

However, the dependency still needs to be satisfied at development-
time. Adding this to `Gemfile` using `:platform => :ruby_21` does not
appear to work in Ruby 1.9.3 , because it is an invalid platform there.
Thus, use `RUBY_VERSION` to specify curses as a development dependency
for Ruby 2.1.0 onwards.

If you know a better way of doing this, I would be very interested to
hear it. :)

ext/mkrf_conf.rb

@@ -0,0 +1,27 @@
+require 'rubygems/dependency_installer'
+
+# SEE: http://en.wikibooks.org/wiki/Ruby_Programming/RubyGems#How_to_install_different_versions_of_gems_depending_on_which_version_of_ruby_the_installee_is_using
+
+di = Gem::DependencyInstaller.new
+
+begin
+  if RUBY_VERSION >= '2.1'
+    puts "Installing curses because Ruby #{RUBY_VERSION}"
+
+    # BEWARE: repetition; SEE: sidekiq-spy.gemspec
+    di.install "curses", "~> 1.0"
+  else
+    puts "Not installing curses because Ruby #{RUBY_VERSION}"
+  end
+rescue => e
+  warn "#{$0}: #{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" + $/)
+end

sidekiq-spy.gemspec

@@ -18,10 +18,24 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]
 
+  spec.extensions += [
+    'ext/mkrf_conf.rb',
+  ]
+
   spec.add_dependency "sidekiq", "~> 2.15"
-  spec.add_dependency "curses", "~> 1.0"
+  # spec.add_dependency "curses", "~> 1.0" # SEE: ext/mkrf_conf.rb
 
   spec.add_development_dependency "bundler", "~> 1.3", "!= 1.5.0"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "mocha", "~> 0.14"
+
+  if RUBY_VERSION >= '2.1'
+    # This is only for development; note that RUBY_VERSION is evaluated at
+    # build-time, not install-time. ext/mkrf_conf.rb takes care of the install-
+    # time dependency; this takes care of the development-time dependency.
+    # Note that this doesn't work in Gemfile, since :ruby_21 is not a valid
+    # platform in Ruby 1.9.3 .
+    # BEWARE: repetition; SEE: ext/mkrf_conf.rb
+    spec.add_development_dependency "curses", "~> 1.0"
+  end
 end