Replace fDATIME with ANSI/POSIX clock_gettime(). Rip out DOS/AMIGA shims.

The thinking here is that we simplify life by going pure ANSI/POSIX.
This is a text game.  If it ever runs on anything but Unix again it's
almost certain to be on something like WSL that supplies a
POSIX-conformant text console.

Makefile

@@ -1,6 +1,6 @@
 # Makefile for the open-source release of adventure 2.5
 
-OBJS=main.o init.o actions1.o actions2.o score.o misc.o datime.o
+OBJS=main.o init.o actions1.o actions2.o score.o misc.o
 SOURCES=$(OBJS:.o=.c) COPYING NEWS README advent.text control
 
 .c.o: