build: Probe MSG_DONTWAIT in the same way as MSG_NOSIGNAL

Instead of the WIN32-specific workaround, detect lack of `MSG_DONTWAIT`
in the build system. This allows other platforms without `MSG_DONTWAIT`
to work too.

src/compat.h

@@ -47,9 +47,7 @@
 #include <unistd.h>
 #endif
 
-#ifdef WIN32
-#define MSG_DONTWAIT        0
-#else
+#ifndef WIN32
 typedef u_int SOCKET;
 #include "errno.h"
 #define WSAGetLastError()   errno
@@ -79,6 +77,11 @@ typedef u_int SOCKET;
 #define MSG_NOSIGNAL 0
 #endif
 
+// MSG_DONTWAIT is not available on some platforms, if it doesn't exist define it as 0
+#if !defined(HAVE_MSG_DONTWAIT) && !defined(MSG_DONTWAIT)
+#define MSG_DONTWAIT 0
+#endif
+
 #if HAVE_DECL_STRNLEN == 0
 size_t strnlen( const char *start, size_t max_len);
 #endif // HAVE_DECL_STRNLEN