Run Travis tests also under i586-mingw32msvc-gcc compiler
Beware that i586-mingw32msvc-gcc is sometimes just a symlink to
i686-w64-mingw32-gcc. Real i586-mingw32msvc-gcc compiler is available in
mingw32 package only in dist trusty.
Add cmake building support to Travis CI
To make it easier to extend the CI support and to be able to execute
the contained tests locally, a dedicated shell script tools/ci_build.sh
was added to execute the tests.