Upstream switched to meson for their build system so do the same as
it will be easier to find problems sooner than later.
This also switch to python3, python is only used for the build.
There is more thing enabled by default (or auto-enabled based on some
configuration) so we need to disable a lot more things than with autotools.
This should be a non functional change.
Reviewed by: zeising (x11@)
Differential Revision: https:/reviews.freebsd.org/D25092
899ab84a70