stdenv/darwin: move secure-format patch to gnum4

This is the only derivation that needs it anymore.

pkgs/development/tools/misc/gnum4/darwin-secure-format.patch

@@ -0,0 +1,15 @@
+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.
+
+--- a/lib/vasnprintf.c	2017-06-22 15:19:15.000000000 -0700
++++ b/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';

pkgs/development/tools/misc/gnum4/default.nix

@@ -13,7 +13,8 @@ stdenv.mkDerivation rec {
   configureFlags = [ "--with-syscmd-shell=${stdenv.shell}" ];
 
   # Upstream is aware of it; it may be in the next release.
-  patches = [ ./s_isdir.patch ] ++ stdenv.lib.optional stdenv.hostPlatform.isDarwin stdenv.secure-format-patch;
+  patches = [ ./s_isdir.patch ]
+    ++ stdenv.lib.optional stdenv.isDarwin ./darwin-secure-format.patch;
 
   meta = {
     homepage = https://www.gnu.org/software/m4/;