[meson] allow building emper on systems without libnuma

Since numa is only needed by emper for a single configuration
and the Debian running on our bigboxes don't have a pkg-config
file for libnuma make it an optional dependency.

Detect libnuma availability in meson and use a wrapper header
defining the needed functions if it is not.
40 jobs for make-libnuma-optional in 6 minutes and 44 seconds (queued for 5 seconds)