config: upgrade to util-macros 1.8 for additional man page support

Use MAN_SUBST now supplied in XORG_MANPAGE_SECTIONS
The value of MAN_SUBST is the same for all X.Org packages.

Use AC_PROG_SED now supplied by XORG_DEFAULT_OPTIONS

Signed-off-by: Gaetan Nadon <memsize@videotron.ca>