freebsd-games: hack needs -fwritable-strings

[James Cook]

$ uname -a
FreeBSD glider 6.3-PRERELEASE FreeBSD 6.3-PRERELEASE #0: Tue Dec 11 13:40:40 PST 2007     james at angel:/usr/obj/usr/src/sys/GLIDER  i386

The problem:
- Install games/freebsd-games
- run hack
- Say "y" when it asks if you're experienced.
- Type "T" for tourist.
There will be a bus error.

This seems to be caused by line 170 of hack.u_init.c, which attempts to
modify a string constant.

SOLUTION:
Add -fwritable-strings to CFLAGS in hack's Makefile.  (There's already
a patch that changes that line of the Makefile, so this is easy to
change.)