Add liquid c if it's available.

jekyll.gemspec

@@ -63,6 +63,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency('jekyll_test_plugin')
   s.add_development_dependency('jekyll_test_plugin_malicious')
   s.add_development_dependency('rouge', '~> 1.7')
+  s.add_development_dependency('liquid-c', '~> 0.0.2')
   s.add_development_dependency('minitest') if RUBY_PLATFORM =~ /cygwin/
   s.add_development_dependency('test-unit') if RUBY_PLATFORM =~ /cygwin/
 end

lib/jekyll.rb

@@ -29,6 +29,12 @@ require 'liquid'
 require 'kramdown'
 require 'colorator'
 
+# Conditional optimizations
+begin
+  require 'liquid-c'
+rescue LoadError
+end
+
 SafeYAML::OPTIONS[:suppress_warnings] = true
 Liquid::Template.error_mode = :strict