correctly deduce p10k directory when using zprezto with zsh 5.1

see #174

powerlevel9k.zsh-theme

@@ -17,36 +17,16 @@ else
   local __p9k_restore_aliases=0
 fi
 
+typeset -g __p9k_root_dir=${POWERLEVEL9K_INSTALLATION_DIR:-${${(%):-%x}:A:h}}
+
 () {
   emulate -L zsh
   setopt no_hist_expand extended_glob no_prompt_{bang,subst} prompt_{cr,percent,sp}
-
   if (( $+__p9k_sourced )); then
     prompt_powerlevel9k_setup
     return
   fi
   typeset -gr __p9k_sourced=1
-  typeset -g __p9k_root_dir=''
-
-  if [[ -n $POWERLEVEL9K_INSTALLATION_DIR ]]; then
-    __p9k_root_dir=${POWERLEVEL9K_INSTALLATION_DIR:A}
-  else
-    if [[ ${(%):-%N} == '(eval)' ]]; then
-      if [[ $0 == '-antigen-load' && -r powerlevel9k.zsh-theme ]]; then
-        # Antigen uses eval to load things so it can change the plugin (!!)
-        # https://github.com/zsh-users/antigen/issues/581
-        __p9k_root_dir=$PWD
-      else
-        >&2 print -P '%F{red}[ERROR]%f Powerlevel10k cannot figure out its installation directory.'
-        >&2 print -P 'Please set %F{green}POWERLEVEL9K_INSTALLATION_DIR.%f'
-        return
-      fi
-    else
-      __p9k_root_dir=${${(%):-%x}:A:h}
-    fi
-  fi
-
-  typeset -gr __p9k_root_dir
   source $__p9k_root_dir/internal/p10k.zsh || true
 }