configure.in: negative time_t for mingw

* configure.in: mingw also uses MSVCRT accepts negative time_t.
  c.f. r58681.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58730 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -1318,6 +1318,7 @@ main()
 		    ac_cv_func_round=no
 		])
 		ac_cv_func_tgamma=no
+		rb_cv_negative_time_t=yes
 		AC_CHECK_TYPE([NET_LUID], [], [],
 			      [@%:@include <winsock2.h>
 			      @%:@include <iphlpapi.h>])