update lfs detection

Author: skywind3000

z.lua

@@ -2691,20 +2691,24 @@ end
 -- LFS optimize
 -----------------------------------------------------------------------
 os.lfs = {}
-os.lfs.disable = os.getenv('_ZL_DISABLE_LFS')
-if os.lfs.disable == nil then
-	os.lfs.status, os.lfs.pkg = pcall(require, 'lfs')
-	if os.lfs.status then
-		local lfs = os.lfs.pkg
-		os.path.exists = function (name)
-			return lfs.attributes(name) and true or false
-		end
-		os.path.isdir = function (name)
-			local mode = lfs.attributes(name)
-			if not mode then 
-				return false
+os.lfs.enable = os.getenv('_ZL_USE_LFS')
+os.lfs.enable = '1'
+if os.lfs.enable ~= nil then
+	local m = string.lower(os.lfs.enable)
+	if (m == '1' or m == 'yes' or m == 'true' or m == 't') then
+		os.lfs.status, os.lfs.pkg = pcall(require, 'lfs')
+		if os.lfs.status then
+			local lfs = os.lfs.pkg
+			os.path.exists = function (name)
+				return lfs.attributes(name) and true or false
+			end
+			os.path.isdir = function (name)
+				local mode = lfs.attributes(name)
+				if not mode then 
+					return false
+				end
+				return (mode.mode == 'directory') and true or false
 			end
-			return (mode.mode == 'directory') and true or false
 		end
 	end
 end