Helper for link window's library

lib/mruby/gem.rb

@@ -106,6 +106,15 @@ module MRuby
         define_gem_init_builder if @generate_functions
       end
 
+      def for_windows?
+        if build.kind_of?(MRuby::CrossBuild)
+          return %w(x86_64-w64-mingw32 i686-w64-mingw32).include?(build.host_target)
+        elsif build.kind_of?(MRuby::Build)
+          return ('A'..'Z').to_a.any? { |vol| Dir.exist?("#{vol}:") }
+        end
+        return false
+      end
+
       def add_dependency(name, *requirements)
         default_gem = requirements.last.kind_of?(Hash) ? requirements.pop : nil
         requirements = ['>= 0.0.0'] if requirements.empty?