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
The existing statement can now be removed from the configuration file.
Use Automake $() for variables in Makefile.am
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>