Fix zcompile race condition

sorin-ionescu/prezto#2028

runcoms/zlogin

@@ -10,7 +10,10 @@
   # Compile the completion dump to increase startup speed.
   zcompdump="${XDG_CACHE_HOME:-$HOME/.cache}/prezto/zcompdump"
   if [[ -s "$zcompdump" && (! -s "${zcompdump}.zwc" || "$zcompdump" -nt "${zcompdump}.zwc") ]]; then
+    if command mkdir "${zcompdump}.zwc.lock" 2>/dev/null; then
       zcompile "$zcompdump"
+      command rmdir  "${zcompdump}.zwc.lock" 2>/dev/null
+    fi
   fi
 } &!