system to build the README file at make dist time
- in util/macros/xorg-macros.m4, add a new XORG_CHECK_LINUXDOC macro that
    will check if the required tools and files exist, and if so set a
    conditional.
- util/modular/symlink.sh
- Link all the <driver>.sgml to xf86-video-<driver>/README.sgml
- Add all the README.<driver> to the list of excluded files
- xc/programs/Xserver/hw/xfree86/doc/sgml/SiS.sgml: Various changes to make
    it spew less warnings when the text file is built.