* lib/mkmf.rb (modified?): return mtime of the target if
  it exists and newer than times.

* lib/mkmf.rb (install_files): add a current directory
  file even if it does not exist yet.

* lib/mkmf.rb (configuration): do not add $LDFLAGS to
  DLDFLAGS.

* ext/extmk.rb (extmake): check whether Makefile is newer
  than depend and MANIFEST.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@4089 b2dd03c8-39d4-4d8f-98ff-823fe69b080e