fix build faild on Cygwin Environment

Fix build faild on Cygwin Environment.

A path on Cygwin like this: */cygdrive/k/work/mruby/include*, but ENV['OS'] is 'Windows_NT'.
So, the path will be replaced by *\cygdrive/k\work\mruby\include* in building process, GCC on Gygwin cannot find this path.

`@file_separator = '\\'` is unnecessary for Windows_NT case. 
Because `@file_separator = '/'` work well on Windows.