Hard-link executable files to mae runnable

As `$ORIGIN` on Linux is refered from the real path of the
executable file, symbolic linked executable file cannot work.

tool/mkrunnable.rb

@@ -53,6 +53,15 @@ end
 
 alias ln_dir_safe ln_safe
 
+case RUBY_PLATFORM
+when /linux/
+  def ln_exe(src, dest)
+    ln(src, dest, force: true)
+  end
+else
+  alias ln_exe ln_safe
+end
+
 if !File.respond_to?(:symlink) && /mingw|mswin/ =~ (CROSS_COMPILING || RUBY_PLATFORM)
   extend Mswin
 end
@@ -80,10 +89,11 @@ def relative_path_from(path, base)
   end
 end
 
-def ln_relative(src, dest)
+def ln_relative(src, dest, executable = false)
   return if File.identical?(src, dest)
   parent = File.dirname(dest)
   File.directory?(parent) or mkdir_p(parent)
+  return ln_exe(src, dest) if executable
   clean_link(relative_path_from(src, parent), dest) {|s, d| ln_safe(s, d)}
 end
 
@@ -116,9 +126,9 @@ ruby_install_name = config["ruby_install_name"]
 rubyw_install_name = config["rubyw_install_name"]
 goruby_install_name = "go" + ruby_install_name
 [ruby_install_name, rubyw_install_name, goruby_install_name].map do |ruby|
+  if ruby and !ruby.empty?
     ruby += exeext
-  if ruby and !ruby.empty? and !File.file?(target = "#{bindir}/#{ruby}")
+    ln_relative(ruby, "#{bindir}/#{ruby}", true)
-    ln_relative(ruby, target)
   end
 end
 so = config["LIBRUBY_SO"]