Our own default values go into accompanying macros prefixed with DEF_.

OpenBSD make supports VPATH but not pattern rules. Everything in the new
Makefile should work no matter what make program is used.

The new Makefile should work with the make programs shipped with FreeBSD,
NetBSD and OpenBSD.

The config.mk file is now optional and only needs to be created if one wants to
persistently overwrite default macro values.

Features used in the Makefile that are not yet in the POSIX standard:

  - Advanced macro assignment operators '+=' and '?=' [1]
  - Special target .PHONY [2]
  - Pattern rules [3]; only needed when $srcdir != '.'. For every pattern rule
    there is an inference rule with the same effect. Hopefully, the inference
    rules get picked up by make programs not supporting pattern rules.
  - Silently including multiple possibly not exisiting files [4] [5]

[1] http://austingroupbugs.net/view.php?id=330
[2] http://austingroupbugs.net/view.php?id=523
[3] http://austingroupbugs.net/view.php?id=513
[4] http://austingroupbugs.net/view.php?id=333
[5] http://austingroupbugs.net/view.php?id=518

None of the mouse mappings uses a keyboard modifier, making it possible to
access the most basic features by only using the mouse.

Next/previous image with left button depending on cursor position, middle
button for dragging, right button for switching to thumnail mode and wheel for
zooming.

Users can keep the old behaviour by simply not adapting the changes to the
buttons array in config.def.h to their config.h file.

This makes mouse panning more direct and faster.

Verbose when build with `V=1'.

Fixes issue #167. Long after closing this issue (kind of wontfix) we have switched to
GNU make and can thus use its features like VPATH.

Misbehaving command lines in the key-handler that rewrite the given files and
thus replace symbolic links instead of their targets shall call realpath(1).

Fixes issue #205 and reverts commit 92e3b578.

Related to issue #253, which originally asked for a non-blocking key-handler.

Use PNG if image has alpha layer, JPG if not. Fixes issue #209

Programs called by the key handler might replace symlinks instead of their
targets.