patch 8.0.0280: problem setting multi-byte environment var on MS-Windows

Problem:    On MS-Windows setting an environment variable with multi-byte
            strings does not work well.
Solution:   Use wputenv when possible. (Taro Muraoka, Ken Takata)
diff --git a/src/vim.h b/src/vim.h
index abee386..f3e87fe 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -594,6 +594,7 @@
 #  endif
 #  define textdomain(domain) (*dyn_libintl_textdomain)(domain)
 #  define libintl_putenv(envstring) (*dyn_libintl_putenv)(envstring)
+#  define libintl_wputenv(envstring) (*dyn_libintl_wputenv)(envstring)
 # else
 #  include <libintl.h>
 #  define _(x) gettext((char *)(x))