native thread prevents runtime shutdown

foo.c

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>


typedef void (*FnPtr) (void);

static pthread_mutex_t mut;
static pthread_cond_t cnd;
static FnPtr d;

static void*
native_starter (void *arg);


void FooStart (FnPtr p) {
	pthread_mutex_init (&mut, (void*)0);
	pthread_cond_init (&cnd, (void*)0);

	d = p;

	printf ("FooStart: starting native thread\n");
	pthread_mutex_lock (&mut);
	pthread_t t;
	pthread_create (&t, (void*)0, native_starter, (void*)0);
	pthread_detach (t);
	pthread_cond_wait (&cnd, &mut);
	pthread_mutex_unlock (&mut);
	printf ("FooStart: native thread signaled us, returning to managed\n");

	pthread_cond_destroy (&cnd);
	pthread_mutex_destroy (&mut);
}

static void*
native_starter (void *arg)
{
	pthread_mutex_lock (&mut);
	printf ("native thread: calling managed\n");
	d ();
	printf ("native thread: returned from managed\n");
	pthread_cond_signal (&cnd);
	pthread_mutex_unlock (&mut);
	//for (int i = 0; i < 2; ++i) {
	while (1) {
		printf ("native thread: sleeping\n");
		sleep (1);
	}
	return (void*)0;
}

Makefile

SOEXT:=dylib
#SOEXT:=so

all: Prog.exe libFoo.$(SOEXT)

.PHONY: all clean

Prog.exe: Prog.cs
	csc $<

libFoo.$(SOEXT): foo.c
	$(CC) -o $@ -shared -g $<

clean:
	-rm -f Prog.exe
	-rm -f libFoo.$(SOEXT)
	-rm -rf libFoo.dylib.dSYM/

Prog.cs

using System;
using System.Threading;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

delegate void MyDelegate ();

public class Prog {

	[DllImport("Foo", EntryPoint="FooStart")]
	extern static void FooStart (MyDelegate d);
	
	public static void Main () {
		var d = new MyDelegate (SomeMethod);
		d ();

		FooStart (d);
		
		// var w = new Thread (() => { d (); });
		//w.Start ();
		// w.Join ();
		return; 
    // Expected result:
    //    we return here
    // Actual result:
    //    we hang waiting for the native thread
	}

	public static void SomeMethod () {
		var t = Thread.CurrentThread;
		Console.WriteLine ("Called from: {0} ({1})", t.ManagedThreadId,
				   t.IsBackground ? "Background" : "Not Background");
	}
}

I tried Mono versions between 6.11 and 4.2.4 on OSX and Linux and the program always hangs.
As far as I can tell this never worked