Various meson updates (including meson tests)

• increase minimal required meson version to 0.64 (so that we can git rid of if-then-else branching in the initial version)
• fix wrong comparison of bool and int types in meson.build
• implement manual meson test suite 'valgrind' to mimic current make valgrind-check (it can be run by: meson test -C your_build_dir --suite=valgrind)

Currently the meson builds will fail due to failing the minimal version requirement. Please update runners to satisfy it