Deal with macOS's libc shenanigans

Fix #9595

reader.lua

@@ -3,11 +3,21 @@
 -- Enforce line-buffering for stdout (this is the default if it points to a tty, but we redirect to a file on most platforms).
 local ffi = require("ffi")
 local C = ffi.C
-ffi.cdef[[
+
+-- macOS is a special snowflake
+if ffi.os == "OSX" then
+    ffi.cdef[[
+extern struct _IO_FILE *__stdoutp;
+void setlinebuf(struct _IO_FILE *);
+]]
+    C.setlinebuf(C.__stdoutp)
+else
+    ffi.cdef[[
 extern struct _IO_FILE *stdout;
 void setlinebuf(struct _IO_FILE *);
 ]]
-C.setlinebuf(C.stdout)
+    C.setlinebuf(C.stdout)
+end
 
 -- Enforce a reliable locale for numerical representations
 os.setlocale("C", "numeric")