man: make substitutions meson-friendly

Change format of substituted variables to follow the pattern used by
configure_file() in Meson.

This helps the migration to Meson, making man/meson.build much cleaner.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>