automake: install binaries to ${bindir} not ${exec_prefix}/games

This has been enforcing an optional part of the FHS that proves
problematic on distributions that do not include /usr/games or
/usr/local/games on the $PATH by default.  On the packagers’ side, the
Arch, Fedora, and OpenBSD packages (at least) have been patching the
Makefile.in files in order to get it to install to the bin directory
instead.  On the users’ side, this comes as a rather nasty surprise
when neither the terminal nor GUI will launch the games when they have
been installed to a location not in the $PATH.

If desired by packagers or end-users, the old behavior can still be
effectively implemented by using `./configure --bindir=/usr/games`

src/Makefile.am

@@ -1,7 +1,7 @@
 
 SUBDIRS = doom heretic hexen strife setup
 
-execgamesdir = ${exec_prefix}/games
+execgamesdir = ${bindir}
 
 execgames_PROGRAMS = @PROGRAM_PREFIX@doom     \
                      @PROGRAM_PREFIX@heretic  \

src/setup/Makefile.am

@@ -1,6 +1,4 @@
 
-gamesdir = $(prefix)/games
-
 AM_CFLAGS = @SDL_CFLAGS@                                           \
             @SDLMIXER_CFLAGS@                                      \
             -I$(top_srcdir)/textscreen -I$(top_srcdir)/src