win32/win32.c: mingw jungle

* configure.in: let mingw do something black-magic, and check if
  _gmtime64_s() is available actually.
* win32/win32.c (gmtime_s, localtime_s): use _gmtime64_s() and
  _localtime64_s() if available, not depending on very confusing
  mingw variants macros.  based on the patch by phasis68 (Heesob
  Park) at [ruby-core:58764].  [ruby-core:58391] [Bug #9119]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44406 b2dd03c8-39d4-4d8f-98ff-823fe69b080e