tests: add targets for building libvirt inside Docker containers

tests: add targets for building libvirt inside Docker containers

The Travis CI system uses Docker containers for its build environment.
These are pre-built and hosted under quay.io/libvirt so that developers
can use them for reproducing problems locally.

Getting the right Docker command syntax to use them, however, is not
entirely easy. This patch addresses that usability issue by introducing
some make targets. To run a simple build (aka 'make all') using the
Fedora 28 container:

   make ci-build@fedora-28

To also run unit tests

   make ci-check@fedora-28

This is just syntax sugar for calling the previous command with a
custom make target

   make ci-build@fedora-28 CI_MAKE_ARGS="check"

To do a purely interactive build it is possible to request a shell

   make ci-shell@fedora-28

To do a MinGW build, it is currently possible to use the fedora-rawhide
image and request a different configure script

   make ci-build@fedora-rawhide CI_CONFIGURE=mingw32-configure

It is also possible to do cross compiled builds via the Debian containers

   make ci-build@debian-9-cross-s390x

In all cases the GIT source tree is cloned locally into a 'ci-tree/src'
sub-directory which is then exposed to the container at '/src'. It is
setup to use a separate build directory so the build takes place in a
subdir '/src/build'. A source tree build can be requested instead
by passing an empty string CI_VPATH= arg to make.

The make rules are kept in a standalone file that is included into the
main Makefile.am, so that it is possible to run them without having to
invoke autotools first.

It is neccessary to disable the gnulib submodule commit check because
this fails due to the way we have manually cloned submodule repos as
primary git repos with their own .git directory, instead of letting
git treat them as submodules in the top level .git directory.

  make[1]: Entering directory '/src/build'
  fatal: Not a valid object name origin
  fatal: run_command returned non-zero status for .gnulib
  .
  maint.mk: found non-public submodule commit
  make: *** [/src/maint.mk:1448: public-submodule-commit] Error 1

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>