Meson

This MR adds the Meson build files. It currently includes the symbol visibility changes to be merged separately in !46. I've not updated the soversion yet as I wasn't sure whether it needed a full version bump (it's currently at 8.0.0). I also set the default C standard to C11 where the autotools script didn't set one, just to hopefully ensure forward compatibility with future compilers which default to a newer standard.