title: Can't run passenger-install-apache2-module
body: <p>now I want to install passenger into my CentOS6.4.</p>
  
  <p>I installed passenger by {gem install passenger}.but I can't run {passenger-install-apache2-module}.because the following error has occured !</p>
  
  <p>please tell me how to fix the problem !</p>
  
  <pre><code>/home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/lib/phusion_passenger/platform_info/apache.rb:188:in ``': No such file or directory - /usr/local/apache2/bin/apxs -q CFLAGS (Errno::ENOENT)
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/lib/phusion_passenger/platform_info/apache.rb:188:in `apache2_module_cflags'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/lib/phusion_passenger/platform_info.rb:110:in `apache2_module_cflags'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/lib/phusion_passenger/platform_info/apache.rb:293:in `apr_config_needed_for_building_apache_modules?'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/lib/phusion_passenger/platform_info.rb:92:in `apr_config_needed_for_building_apache_modules?'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/bin/passenger-install-apache2-module:69:in `dependencies'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/lib/phusion_passenger/abstract_installer.rb:166:in `check_dependencies'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/bin/passenger-install-apache2-module:90:in `install!'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/lib/phusion_passenger/abstract_installer.rb:63:in `start'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/gems/passenger-3.0.19/bin/passenger-install-apache2-module:236:in `&lt;top (required)&gt;'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/bin/passenger-install-apache2-module:19:in `load'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/bin/passenger-install-apache2-module:19:in `&lt;main&gt;'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/bin/ruby_noexec_wrapper:14:in `eval'
      from /home/kenta/.rvm/gems/ruby-1.9.3-p392@global/bin/ruby_noexec_wrapper:14:in `&lt;main&gt;'
  </code></pre>