build-sys: Do not require xsltproc for installation of man pages

Same reason as found in this commit to systemd:

	commit 4ca39b280fce3c60d2fdecbd478fd9bf7f9d3e64
	Author: Mike Gilbert <floppym@gentoo.org>
	Date:   Sun Feb 23 11:21:13 2014 -0500

	    configure: Do not require xsltproc for installation of man pages

	    The release tarballs ship with pre-generated man pages, so we do not
	    need xsltproc for a typical end-user build.

	    Developers will probably have xsltproc anyway, but if not they will now
	    encounter a build-time failure instead of an error in configure.
1 file changed