[Xastir] A few changes

[Curt Mills]

1) After updating from Git you'll see a new message in the configure
summary:

    RECOMMENDED OPTIONS:
      Xpm / Snapshots ........................... : yes

If you have libxpm and libxpm-dev packages installed (the names of the
packages may vary on your system) then you'll see a "yes" there. That
enables the Snapshot facility in Xastir. That was somewhat hidden before,
perhaps only greying-out the button in the menus if it wasn't present
during configure/compile.

2) In the Help->About dialog you'll see at the very top something like:

    Xastir V2.1.1 (cf75dd5a)

That last number is the 8-digit short-form version of the most recent Git
commit. That tells you _exactly_ which code is in your compile.

3) You may notice an absence or a severe reduction in compile warnings.
This is a result of multiple people working over the last 1.5 weeks on this
particular issue (Issue #24). We still have a few warnings related to
geotiff / graphicsmagick / imagemagick: These appear to be problems in
their headers, not our code. With a very new GCC compiler (7 or 8?) you may
also see a bunch of messages about snprintf but there exist some compiler
flags which will quiet those: We're using them in one of our Travis-CI
configs to do just that.

4) While working on #3 above we found some real bugs and worked to fix them
or in some cases add them to our bug list.

5) Travis-CI is building Xastir every time there is a commit or a pull
request. It currently builds on three Ubuntu-16.04 systems and one OSX
10.13.3 system and feeds back warnings/errors to us. You can see this at:
https://travis-ci.org/Xastir/Xastir
Thanks to Jason Godfrey for setting this up. We're worked quite a bit on
the configuration in the last week to make it do what we want.

6) The project is currently at zero pull requests and only a few
outstanding bugs.
https://github.com/Xastir/Xastir

-- 
Curt, WE7U        http://we7u.wetnet.net        http://www.sarguydigital.com