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/build.rb

@@ -1,6 +1,7 @@
 require "mruby/core_ext"
 require "mruby/build/load_gems"
 require "mruby/build/command"
+autoload :Find, "find"
 
 module MRuby
   autoload :Gem, "mruby/gem"
@@ -487,6 +488,20 @@ EOS
       @internal
     end
 
+    def each_header_files(&block)
+      return to_enum(__method__) unless block
+
+      basedir = File.join(MRUBY_ROOT, "include")
+      Find.find(basedir) do |d|
+        next unless File.file? d
+        yield d
+      end
+
+      @gems.each { |g| g.each_header_files(&block) }
+
+      self
+    end
+
     protected
 
     attr_writer :presym

lib/mruby/core_ext.rb

@@ -22,6 +22,27 @@ class String
   def remove_leading_parents
     Pathname.new(".#{Pathname.new("/#{self}").cleanpath}").cleanpath.to_s
   end
+
+  def replace_prefix_by(dirmap)
+    [self].replace_prefix_by(dirmap)[0]
+  end
+end
+
+class Array
+  # Replace the prefix of each string that is a file path that contains in its own array.
+  #
+  # dirmap is a hash whose elements are `{ "path/to/old-prefix" => "path/to/new-prefix", ... }`.
+  # If it does not match any element of dirmap, the file path is not replaced.
+  def replace_prefix_by(dirmap)
+    dirmap = dirmap.map { |older, newer| [File.join(older, "/"), File.join(newer, "/")] }
+    dirmap.sort!
+    dirmap.reverse!
+    self.flatten.map do |e|
+      map = dirmap.find { |older, newer| e.start_with?(older) }
+      e = e.sub(map[0], map[1]) if map
+      e
+    end
+  end
 end
 
 def install_D(src, dst)

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