Use uppercase for externally defined macros and lowercase for macros defined in
the makefile.

Also simplify generation of version.h.

Since d8114e8 the file name is used for caching as-is without adding
.jpg at the end, but sxiv -c still expected old format causing it to
remove all fresh thumbnails.

This also sets *markidx* when using the right mouse button in thumbnail mode.

*Shift-m* feels much more natural for *mark_range* and *Ctrl-u* is decent
enough for *unmark_all*.

Sets the marked state of all images ranging from the latest marked/unmarked
image to the current image, to the state of that latest toggled image.

VERSION string between releases is last release suffixed with '+'.
Additionally, use output of git-describe instead of VERSION string, if it is
not empty.

Fixes issue #308.

Related to issue #303

Before that zoomdiff was a merely heuristic.

Related to issue #303

Fixes issue #302

When sxiv is built with AUTORELOAD=nop if something is feed via stdin it
will start to infinitely call select() leading to a 100% CPU usage.

Thanks to @sdx23 for helping in analyzing that problem!

Fixes issue #297.

Non-fitting text is truncated by simply cutting it off.

win_textwidth() has been replaced by a simple macro wrapper around
win_draw_text() with a maximum width of zero, which results in a dry-run that
still calculates the number of pixels used by the text, much like snprintf(3)
with a buffer length of zero.

Code under a different license should be kept in a separate file. This
implemention is a single header file with ~65 lines, so it better fits this
requirement.

Fixes #276

Instead of rendering the entire filename at once, Xft will let us do it
character by character. This will allow sxiv to query fontconfig for
a font that can provide any missing codepoints, if needed.

A known issue of this patch is that the "..." dots rendering will not
work properly for very long multibyte filenames. That is because we
cannot easily predict the final width of the rendered filename before
drawing it. I couldn't figure out a clean way to deal with this, so I
ended up just truncating the offending filenames.

Fixes issue #295

The imlib2 TGA loader returns an imlib image handle without any actual data
when given a text file like this:

    T
    Content-Type: application/javascript
    Content-Length: 3836
    Last-Modified: Wed, 23 Sep 2015 12:25:47 GMT
    Etag: "56029a4b-efc"
    Expires: Sat, 20 Aug 2016 15:14:33 GMT
    Cache-Control: max-age=604800, public
    Accept-Ranges: bytes

Fortunately, `imlib_image_get_data()` returns NULL in this case, so that we can
use it as an additional check when opening files.

Fixes issue #293.

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