[build] Add cmake build system.

This is the merge commit of the development branch `cmake-transition`.

This changes makes sure that:

1. `liblseef`, `libsleefgnuabi`, and `libdft` are build with the Makefile system;
2. tests for `libdft` are executed with the Makefile system;
3. `libsleef` and `libsleefgnuabi` are built and tested with cmake.

Notes:
1. `libdft` is build on linux only for gcc.
2. The makefile build of `libsleef` is done only for linux/gcc.
3. The `ENABLE_WEAK_ALIASES` C macro has been replaced with
    `ENABLE_GNUABI` to handle with the same maro both cmake
    and Makefile build systems.
4. A change log file has been added. This will be used to keep track
    of the changes. It uses the format specified in
    http://keepachangelog.com/en/1.0.0/

5. The macro `ENABLE_GNUABI` is set up as a consequence of the
    configure time check `COMPILER_SUPPORTS_WEAK_ALIASES`.
    The `SLEEF_ENABLE_GNUABI` cmake variable has been replaced
    with `ENABLE_GNUABI`.