[meson] Add ragel subproject

If ragel 6.10 is not found, build it from source.

Seems to work, except that ragel uses exceptions and we configure
HarfBuzz build to not use exceptions, and I can’t find away to enable
exceptions only for the ragel subproject. I had to remove cpp_eh=none
from default options and try to disable exceptions in MSVC manually
(other compilers are already handled).
7 files changed