Some improvements to the build system

1) Instead of copying chocolate-setup into chocolate-{doom,heretic,hexen,strife}-setup,
   respectively, check in configure for symbolic linking and use that if available
   (falls back to "cp -pR" if not).
2) Add $(icons_DATA) to CLEANFILES so they get removed in clean phase; they are
   generated from doom-png and setup.png, respectively.
3) Consistently use @PROGRAM_PREFIX@ instead of the hard-coded "chocolate-" in
   the rules for the man pages.
4) Generate one man page for each setup program by symbolic linking of
   chocolate-setup.6 (see 1), add them to the list of installed man pages and
   to CLEANFILES.
5) Remove the redundant $(EXEEXT) suffix from the setup binaries (they were missing
   for execgames_PROGRAMS, anyway)