configure.bat: --with-libdir

	* win32/configure.bat: add --with-libdir option for basename of
  libdir.  on Windows it must be placed under exec_prefix always.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@54608 b2dd03c8-39d4-4d8f-98ff-823fe69b080e