meson: use absolute paths in manpage substitutions

paths returned by get_option('foodir') are potentially relative to prefix

Noticed when comparing manpages generated by a meson build with those
generated by an autotools build

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>