Added multiple CMake-related files to project.
Supports building the library and the tests.
See CMakeLists.txt for notes on how it works.
I had to adjust 3 existing files in order to disable some configuration
that should be taken care of by cmake/automake anyway.
I also added jansson.def from a future jansson version,
to test cmake's support for .def files (which works fine).
3e03b07831