Copy header files to the build directory

This is so that the build directory can be regarded as a temporary installation directory to work in.

Directories set in `MRuby::Gem::Specification#export_include_paths` are used as they are if they are subdirectories placed under `gem.dir`.
Files are placed under `<build-dir>/include/mruby/gems/<gem-name>/<gem-include_paths>` to separate each GEM.

When GEM is compiled by building `libmruby.a`, the same `include_paths` will be set as before, so it is expected that no unintended references will be made.

lib/mruby/gem.rb

@@ -292,6 +292,19 @@ module MRuby
           end
         end.all?
       end
+
+      def each_header_files(&block)
+        return to_enum(__method__) unless block
+
+        self.export_include_paths.flatten.uniq.compact.each do |dir|
+          Find.find(dir) do |d|
+            next unless File.file? d
+            yield d
+          end
+        end
+
+        self
+      end
     end # Specification
 
     class Version