Fix a bug that make.exe path may be broken

GitHub: fix GH-225

This is not a real fix. This works only when Windows user doesn't
specify make.exe path explicitly by MAKE environment variable.

If an user specifies c:\...\make.exe by MAKE environment variable
explicitly, it still doesn't work. But I'm not sure whether the use
case exists or not. If we get a report for the use case and the user
can't use c:/.../make.exe path instead of c:\...\make.exe, I'll
reconsider how to fix the use case.

Reported by Lukasz Suleja. Thanks!!!

lib/rake/extensiontask.rb

@@ -513,12 +513,15 @@ def make
     def find_make
       candidates = ["gmake", "make"]
       paths = (ENV["PATH"] || "").split(File::PATH_SEPARATOR)
+      paths = paths.collect do |path|
+        Pathname(path).cleanpath
+      end
 
       exeext = RbConfig::CONFIG["EXEEXT"]
       candidates.each do |candidate|
         paths.each do |path|
-          make = File.join(path, "#{candidate}#{exeext}")
-          return make if File.executable?(make)
+          make = path + "#{candidate}#{exeext}"
+          return make.to_s if make.executable?
         end
       end