add travis-ci.org build machinery

add travis-ci.org build machinery

The idea is that .travis.yml is specific to Travis-CI, but most of the
actual work is done in tools/ci-build.sh, which should be reasonably
CI-platform-agnostic (it currently assumes that build-dependendencies are
preinstalled, that the "native" platform we're building on is GNU/Linux
or something very close, and that "mingw" means mingw-w64 as packaged
in Debian and Ubuntu).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=93194
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>