Add meson as alternative build system

Using a build system like meson has some advantages over autotools: Proper out-of-tree builds, generation of a compile_commands.json that simplifies the use of tools like clang-tidy, automatic parallel compilation, and the mind-share of meson is growing.

My autotools-fu is a bit rusty, so the translation from autotools to meson might not be perfect yet. Add a note in the readme-file that autotools is still the default build system.