Documentation: use "spurious .sp" XSLT if DOCBOOK_SUPPRESS_SP is set

Documentation: use "spurious .sp" XSLT if DOCBOOK_SUPPRESS_SP is set

With this change, the "spurious .sp" suppression XSLT code is
disabled by default. It can be enabled by defining
DOCBOOK_SUPPRESS_SP.

The "spurious .sp" XSLT fragment was used to work around a bug
first released in docbook-xsl 1.69.1. Modern versions of
docbook-xsl are negatively affected by the code (some empty lines
are omitted from manpage output; see
<http://article.gmane.org/gmane.comp.version-control.git/115302>).

The key revisions in the docbook SVN repo seem to be 5144 (before
docbook-xsl 1.69.1) and 6359 (before docbook-xsl 1.71.1).

Testing done with asciidoc 8.3.1 and docbook-xsl 1.74.0.

Signed-off-by: Chris Johnsen <chris_johnsen@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>