patch 8.1.2344: Cygwin: warning for using strptime()

Problem:    Cygwin: warning for using strptime().
Solution:   Move defining _XOPEN_SOURCE and __USE_XOPEN to vim.h. (Ken Takata,
            closes #5265)  Use 700 for _XOPEN_SOURCE for mkdtemp().

src/vim.h

@@ -36,8 +36,21 @@
     Error: configure did not run properly.  Check auto/config.log.
 # endif
 
+# ifdef UNIX
+// Needed for strptime().  Needs to be done early, since header files can
+// include other header files and end up including time.h, where these symbols
+// matter for Vim.
+// 700 is needed for mkdtemp().
+#  ifndef _XOPEN_SOURCE
+#   define _XOPEN_SOURCE    700
+#  endif
+#  ifndef __USE_XOPEN
+#   define __USE_XOPEN
+#  endif
+# endif
+
 // for INT_MAX, LONG_MAX et al.
-#include <limits.h>
+# include <limits.h>
 
 /*
  * Cygwin may have fchdir() in a newer release, but in most versions it