provide linux build instructions in the source distro

sys/unix/README.linux

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+NetHack 3.4.0 Linux Elf
+
+This README provides the instructions for using the official Linux binary,
+system platform requirements, as well as steps used to create that binary.
+The same steps can be used from the source distribution to create a similar
+binary.
+
+The official Linux binary has support for tty and X11 windowing systems, but
+not Qt.  This means you will need to have X11 libraries installed on your
+system to run this binary, even in its tty flavor.
+
+
+The Linux binary package assumes that you have a user and a group named
+"games" on your system.  If you do not, you can simplify installation by
+creating them first.
+
+gunzip and untar the package, preserving permissions, from / to put the
+NetHack files in /usr/games/nethack and /usr/games/lib/nethackdir.
+
+(If you have old record and logfile entries from a previous NetHack
+version, you might want to save copies before they get overwritten by the
+new empty files; old saved games and bones files won't work with 3.4.0).
+
+In addition to data files for running the game, you will find other useful
+things in /usr/games/lib/nethackdir (such as a copy of this README :-).
+
+The general documentation Guidebook.txt and the processed man pages
+nethack.txt and recover.txt should provide an introduction to the game.
+
+The sample config file called dot.nethackrc can be used by copying
+it to your home directory as .nethackrc and modifying it to your liking.
+
+If you are running X11 copy the nh10.pcf and ibm.pcf font files from
+/usr/games/lib/nethackdir to a X11 fonts directory (such as
+/usr/X11/lib/X11/fonts/misc) and run "mkfontdir", then restart X
+windows to load them.  Also consider putting NetHack.ad in
+/usr/X11/lib/X11/app-defaults or its contents in your .Xdefaults or
+.Xresources.
+
+
+The official Linux binary is set up to run setgid games, which allows
+multiple users on your system to play the game and prevents cheating by
+unprivileged users.  The usual default for NetHack is setuid games, but
+this causes problems with accessing .nethackrc on distributions with
+restrictive default security on home directories and users who don't know
+the tradeoffs of various permission settings.
+
+
+If you have problems, send us some email.
+
+nethack-bugs@nethack.org
+
+
+
+Steps used to build this binary release
+
+System:  egcs-1.1.2, XFree86-3.3.6, ncurses-5.0, glibc-2.1.3
+
+0.  Makefile.top: GAMEGRP = games
+		  GAMEPERM = 02775
+		  FILEPERM = 0664
+		  EXEPERM = 0775
+		  DIRPERM = 0775
+		  VARDATND = x11tiles pet_mark.xbm rip.xpm
+
+1.  Makefile.src: define LINUX and linux options throughout
+		  WINSRC = $(WINTTYSRC) $(WINX11SRC)
+		  WINOBJ = $(WINTTYOBJ) $(WINX11OBJ)
+		  WINTTYLIB = /usr/lib/libncurses.a
+                  Include Xpm libs in WINX11LIB
+		  WINLIB = $(WINTTYLIB) $(WINX11LIB)
+
+2.  Makefile.utl: define LINUX and linux options throughout
+                  Use bison/flex instead of yacc/lex
+
+3.  config.h: define X11 window support, XPM support.
+              define COMPRESS as /bin/gzip as that is where it
+              seems to reside on newer Linux's.
+              define COMPRESS_EXTENSION as ".gz"
+              define DLB
+
+4.  unixconf.h: define LINUX
+                define TIMED_DELAY
+
+5.  make all; (cd util; make recover); su; make install;
+    cp util/recover /usr/games/lib/nethackdir/recover
+
+6.  Convert nh10.bdf and ibm.bdf to proper font files and place in
+    font path.