travis-ci: refactor and add .travis-functions.sh

Travis yaml syntax, where we can only use shell one-liners, is awful
and ugly. We add a real shell script and source it from .travis.yml.

This commit squashes a lot changes because we don't want to pollute
history with this meta CI stuff.

Highlights of this commit:
  - enable make distcheck
  - cleanup configure options (enable all progs, with python and docs)
  - shorter config.log dump
  - out-of-tree build supported
  - workaround broken libtool on travis host
  - now it's easy to add temporary debugging stuff
  - testing locally is possible now

What are we testing now:
  - gcc and clang compiler
  - configure mostly with --enable-all-programs, --with-python and
    --enable-gtk-doc
  - make check with and without root permissions
  - make distcheck
  - make install (with and without --prefix as well as DESTDIR=...)
  - usually we always build in-tree but distcheck does out-of-tree

What do we want/expect at all:
  - This travis setup is intended to always work without fixing or
    updating this build script all the time. Thats why we only use a
    few configure flags.
  - We want to avoid predictable failures on travis host to not make
    developers tired of this machinery. Any tested feature which is
    known to be (sometimes) broken should be fixed or skipped as soon
    as possible.
  - If this works like wanted then in future almost any point in master
    commit history should at least survive the build and common tests
    on this reference host/arch. Moreover we will have a comparable,
    deterministic build log history. This could make bisecting real
    bugs easier.
  - It's not our goal to test any kind of config flags, host setup and
    arch combination. This would be done better by external build
    scripts which do not live within the project itself.
  - Of course this machinery can be also used to reproduce bugs with
    more exotic config and host setup by pushing a debug branch with
    modified travis script to github.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>