luacheck: enable cache (#2004)

* luacheck: enable cache

* Cleanup .gitignore: remove .build* and awesome

We only use the "build" dir by now, where also the "awesome" binary gets
created.

.gitignore

@@ -2,5 +2,4 @@
 .version_stamp
 .*.sw?
 build
-.build*
-/awesome
+.luacheckcache

.luacheckrc

@@ -46,4 +46,7 @@ globals = {
     "client"
 }
 
+-- Enable cache (uses .luacheckcache relative to this rc file).
+cache = true
+
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80