build: avoid error message during autogen.sh

Do not complain if the directory already exists.

mkdir: cannot create directory "autotools": File exists
build: fix not-honored --docdir setting

Despite using `./configure --docdir=XYZ` and/or `make docdir=XYZ`, the
directory is ignored. Resolve that problem.
build: do not create an INSTALL file

If the general-instruction INSTALL file is not desired, just specify so
in AUTOMAKE_OPTIONS rather than fiddling with it in autogen.sh.
build: kill redundant boilerplate in autogen.sh

The auxiliary build directory specified with AC_BUILD_AUX_DIR is
also automatically created AFAICS, so it need not be created
by autogen.sh.

[For directories that are not (such as with AC_CONFIG_MACRO_DIR),
place an empty .gitignore file in it, so that it always exists.]
build: adjust location of execgamesdir

For arch-dependent files, there is $exec_prefix. In most practical
cases, it has the same value as $prefix, but do use it anyhow.
setup: Add key_mission to the controls group.

Otherwise, the same key can be bound to "Show mission" and one of the other
"More controls..." at the same time.
heretic, hexen: Fix mouse (and joystick) weapon cycling.

Implement this the same way it is implemented for doom and strife.
Merge pull request #318 from svdijk/bugfix

Heretic/Hexen/Strife bugfixes from Sander van Dijk.
heretic: fix automap panning left

When follow mode is off, hitting left would cause it to scroll all the way to the west of the map without stopping.
Merge pull request #320 from fielder/master

heretic: fix automap panning left
configure: Add option to disable libsamplerate.

Some distros don't like automagic dependencies. Fixes #315.
doom: Add support for Freedoom IWAD names.

Freedoom has new IWAD names. Support these as fallbacks if no other
IWAD file can be found, but prefer FreeDM as it has Vanilla-
compatible levels. Show a warning on startup if running using the
main Freedoom IWADs.
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)
sound: Fix swapped parameters for function.

CheckVolumeSeparation() has the sep and vol parameters backwards
compared to how they are used when the function is called. Reverse
the order to fix this (thanks proteal). Fixes #330.
Revert the more controversial changes in my previous pull request.

Add back  to binaries that are built and installed as _SCRIPTS targets.
Copy instead of symbolic linking, the files are needed as binary copies anyway to allow for separate packaging of the game engines.
Merge pull request #329 from fabiangreffrath/master

Some improvements to the build system
hires: still major issues with ceiling/floor rendering and psprite scaling
basic rendering works
Fix psprite scaling
fix sky scaling