diff -Naur NetHack-NetHack-3.6.6_Released/include/unixconf.h NetHack-NetHack-3.6.6_Released.serenity/include/unixconf.h
--- NetHack-NetHack-3.6.6_Released/include/unixconf.h	2020-03-08 18:29:31.000000000 +0100
+++ NetHack-NetHack-3.6.6_Released.serenity/include/unixconf.h	2021-04-11 21:54:47.295254864 +0200
@@ -246,8 +246,10 @@
  * Comment out the USE_FCNTL if for some reason you have a strange
  * OS/filesystem combination for which fcntl(2) does not work. */
 #ifdef POSIX_TYPES
+#ifndef __serenity__
 #define USE_FCNTL
 #endif
+#endif
 
 /*
  * The remainder of the file should not need to be changed.