.vimrc: fix library path for clang_complete (on NixOS)
.gitconfig: use ~/.gitignore as a global ignore file

From 'git config --help':

core.excludesfile
  In addition to .gitignore (per-directory) and .git/info/exclude, Git looks into
  this file for patterns of files which are not meant to be tracked. "~/" is
  expanded to the value of $HOME and "~user/" to the specified user’s home
  directory. Its default value is $XDG_CONFIG_HOME/git/ignore. If
  $XDG_CONFIG_HOME is either not set or empty, $HOME/.config/git/ignore is used
  instead. See gitignore(5).