enable event loop by default on macOS

only for the Pd app, not for libpd

mac/README.txt

@@ -249,9 +249,9 @@ the UI thread must be the main thread, otherwise it won't receive any input even
 to a secondary thread and runs a Cocoa event loop on the main thread.
 
 Unfortunately, some externals like 'Gem' or 'ophelia' already run their own (polling)
-event loop and expect the scheduler to run on the main thread. To maintain backwards
-compatibility we can't enable the event loop by default, instead users must request
-it explicitly with the "-eventloop" flag.
+event loop and expect the scheduler to run on the main thread. If you want to use
+these externals, you have to disable the event loop, either by unchecking
+"Enable event loop" in the "Startup" dialog or by starting Pd with "-noeventloop".
 
 For now, the event loop is only implemented for macOS, because there it is essential.
 Win32 and X11, on the other hand, don't have any notion of a "main thread", so externals

src/s_main.c

@@ -342,6 +342,11 @@ int sys_main(int argc, const char **argv)
     const char *prefsfile = "";
     sys_externalschedlib = 0;
     sys_extraflags = 0;
+#ifdef __APPLE__
+    /* on macOS we enable the event loop by default,
+     * but only for the Pd app (not for libpd). */
+    sys_eventloop = 1;
+#endif
 #ifdef PD_DEBUG
     fprintf(stderr, "Pd: COMPILED FOR DEBUGGING\n");
 #endif