Allow building on Haiku OS.

Note: this is not an officially supported target.
Thanks to kallisti5.

src/lj_arch.h

@@ -74,7 +74,7 @@
        defined(__NetBSD__) || defined(__OpenBSD__) || \
        defined(__DragonFly__)) && !defined(__ORBIS__)
 #define LUAJIT_OS	LUAJIT_OS_BSD
-#elif (defined(__sun__) && defined(__svr4__))
+#elif (defined(__sun__) && defined(__svr4__)) || defined(__HAIKU__)
 #define LUAJIT_OS	LUAJIT_OS_POSIX
 #elif defined(__CYGWIN__)
 #define LJ_TARGET_CYGWIN	1