MacOS SDKs before 10.13 don't support open_memstream. This is already replaced
by a runtime check in cctools-port, but because we build with SDK 10.12 by
default, linking still fails for us. Disable it entirely here.

--- a/cctools/include/stuff/diagnostics.h
+++ b/cctools/include/stuff/diagnostics.h
@@ -60,13 +60,6 @@ void diagnostics_log_msg(enum diagnostic_level level, const char* message);
  */
 void diagnostics_write(void);
 
-#if defined(__APPLE__ ) && defined(__has_builtin)
-#  if __has_builtin(__builtin_available)
-#    define HAVE_OPENMEMSTREAM_RUNTIME __builtin_available(macOS 10.13, *)
-#  endif
-#endif
-#ifndef HAVE_OPENMEMSTREAM_RUNTIME
-#  define HAVE_OPENMEMSTREAM_RUNTIME 1
-#endif
+#define HAVE_OPENMEMSTREAM_RUNTIME 0
 
 #endif /* diagnostics_h */