Makefile.sub: refine distclean-ext
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57394 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Makefile.sub: refine distclean-ext
extinit.c and suppress error message when failed to remove ext
directory.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57394 b2dd03c8-39d4-4d8f-98ff-823fe69b080e