Fixes a couple build errors in the following cases:
numpy is installed from somewhere other than Debian, and needs its includedir explicitly passed
cmake is not installed, but fmt/spdlog are available via pkg-config
on systems with too old Meson, raise a better error message. I assume that you need to support at least 0.53.0 due to Ubuntu 20.04, so I left the minimum version as the version that introduced features this project is already using
Fixes a couple build errors in the following cases: