Makefile: add a knob to enable the use of Asciidoctor

Makefile: add a knob to enable the use of Asciidoctor

While Git has traditionally built its documentation using AsciiDoc, some
people wish to use Asciidoctor for speed or other reasons.  Add a
Makefile knob, USE_ASCIIDOCTOR, that sets various options in order to
produce acceptable output.  For HTML output, XHTML5 was chosen, since
the AsciiDoc options also produce XHTML, albeit XHTML 1.1.

Asciidoctor does not have built-in support for the linkgit macro, but it
is available using the Asciidoctor Extensions Lab.  Add a macro to
enable the use of this extension if it is available.  Without it, the
linkgit macros are emitted into the output.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>