title: YouCompleteMe can't autocomplete
body: <p>I want to develop C/C++ programs, so I installed YouCompleteMe for Vim through Vundle.but it can't work normally,  In fact, it shows only the words contained in the current file.hope to help!
  my step are as follow:</p>
  
  <ul>
  <li>download Vundle.vim</li>
  </ul>
  
  <pre>
      git clone https://github.com/gmarik/Vundle.vim.git ~/.vim/bundle/Vundle.vim
  </pre>
  
  <ul>
  <li>#modify .vimrc</li>
  </ul>
  
  <pre>
  
      set nocompatible
      filetype off
      set rtp+=~/.vim/bundle/Vundle.vim
      call vundle#begin()
      Plugin 'gmarik/Vundle.vim'
      Plugin 'Valloric/YouCompleteMe'
      call vundle#end()
      filetype plugin indent on
  
  </pre>
  
  <ul>
  <li>Launch vim and run:</li>
  </ul>
  
  <pre>`:PluginInstall`</pre>
  
  <ul>
  <li>download cmake and clang+llvm</li>
  </ul>
  
  <pre>
  
      http://llvm.org/releases/download.html#3.6.0
      http://www.cmake.org/download/
  
  </pre>
  
  <ul>
  <li>prepare clang and cmake</li>
  </ul>
  
  <pre>
  
      Extract "clang+llvm-3.6.0-x86_64-linux-gnu-ubuntu-14.04.tar.xz" into ycm_temp
      Rename "clang+llvm-3.6.0-x86_64-linux-gnu" to "llvm_root_dir"
      Extract cmake-3.2.2-Linux-x86_64.tar.gz and Link bin/cmake to /usr/bin/cmake
  
  </pre>
  
  <ul>
  <li>make</li>
  </ul>
  
  <pre>
  
      cd ~
      mkdir ycm_build
      cd ycm_build
      cmake -G "Unix Makefiles" -DPATH_TO_LLVM_ROOT=~/ycm_temp/llvm_root_dir . ~/.vim/bundle/YouCompleteMe/third_party/ycmd/cpp
      make ycm_support_libs
  
  </pre>
  
  <ul>
  <li>modify .vimrc</li>
  </ul>
  
  <pre>
  
      let g:ycm_seed_identifiers_with_syntax=1
      let g:ycm_global_ycm_extra_conf = '/home/li/.vim/bundle/YouCompleteMe/.ycm_extra_conf.py'
      let g:ycm_confirm_extra_conf=0
      let g:ycm_collect_identifiers_from_tag_files = 1
      set completeopt=longest,menu
  
  </pre>
  
  <p>Now, No error or warn be thrown, but it can't autocomplete C/C++ header files!</p>
  
  <ul>
  <li>Note</li>
  </ul>
  
  <pre>
  OS:ubuntu 14.04
  vim:7.4
  Python:2.7.6
  </pre>