With format string strictness, High Sierra also enforces that %n isn't used
in dynamic format strings, but we should just disable its use on darwin in
general.

--- lib/vasnprintf.c.orig	2017-06-22 15:19:15.000000000 -0700
+++ lib/vasnprintf.c	2017-06-22 15:20:20.000000000 -0700
@@ -4869,7 +4869,7 @@ VASNPRINTF (DCHAR_T *resultbuf, size_t *
 #endif
                   *fbp = dp->conversion;
 #if USE_SNPRINTF
-# if !(((__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 3)) && !defined __UCLIBC__) || ((defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__))
+# if !defined(__APPLE__) && !(((__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 3)) && !defined __UCLIBC__) || ((defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__))
                 fbp[1] = '%';
                 fbp[2] = 'n';
                 fbp[3] = '\0';