configure.in, win32/Makefile.sub: PACKED_STRUCT with VC

* configure.in (PACKED_STRUCT): check VC pragma too, and make
  function-style macro.

* win32/Makefile.sub (PACKED_STRUCT): enable with VC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45173 b2dd03c8-39d4-4d8f-98ff-823fe69b080e