Meson support added and tested with CI.

Meson support has been added, and testing with CI, including comparing against Autotools has been added too.

Two POSIX.2 utilities have been added for Python support:

- find_vpython.sh: find a python interpreter in the PATH
matching a version condition;

- compile_py.sh: compile the python source files, handling
DESTDIR (removing it from the path included in the
byte-compiled version) and allowing request of several levels
of optimization (0,1 or 2).
NOTE : SOURCE_DATE_EPOCH can be used to embed in the
byte-compiled version a defined date, allowing to byte compare
the versions produced by Autotools and Meson (done in the CI:
see .gitlab-ci.yml).