lexpr: Lua 5.2 compatibility

examples/lexpr.pd_lua

 local lexpr = pd.Class:new():register("lexpr")
 
+-- AG: Lua 5.2 compatibility (cf. https://stackoverflow.com/a/14554565)
+if not setfenv then -- Lua 5.2
+  -- based on http://lua-users.org/lists/lua-l/2010-06/msg00314.html
+  -- this assumes f is a function
+  local function findenv(f)
+    local level = 1
+    repeat
+      local name, value = debug.getupvalue(f, level)
+      if name == '_ENV' then return level, value end
+      level = level + 1
+    until name == nil
+    return nil end
+  getfenv = function (f) return(select(2, findenv(f)) or _G) end
+  setfenv = function (f, t)
+    local level = findenv(f)
+    if level then debug.setupvalue(f, level, t) end
+    return f end
+end
+
 local function sandbox(e, f) -- only supports nullary f() with one return
   local g = getfenv(f)
   setfenv(f, e)