man: make substitutions meson-friendly
This is a small patch from @pq and R-b me, lifted from the Meson MR !8 (merged), making it easier to carry over our manpages to the Meson build.
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