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

Documentation/Makefile

@@ -174,6 +174,18 @@ ifdef GNU_ROFF
 XMLTO_EXTRA += -m manpage-quote-apos.xsl
 endif
 
+ifdef USE_ASCIIDOCTOR
+ASCIIDOC = asciidoctor
+ASCIIDOC_CONF =
+ASCIIDOC_HTML = xhtml5
+ASCIIDOC_DOCBOOK = docbook45
+ifdef ASCIIDOCTOR_EXTENSIONS_LAB
+ASCIIDOC_EXTRA = -I$(ASCIIDOCTOR_EXTENSIONS_LAB) -rasciidoctor/extensions -rman-inline-macro
+endif
+ASCIIDOC_EXTRA += -alitdd='&\#x2d;&\#x2d;'
+DBLATEX_COMMON =
+endif
+
 SHELL_PATH ?= $(SHELL)
 # Shell quote;
 SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))

Makefile

@@ -250,6 +250,12 @@ all::
 # apostrophes to be ASCII so that cut&pasting examples to the shell
 # will work.
 #
+# Define USE_ASCIIDOCTOR to use Asciidoctor instead of AsciiDoc to build the
+# documentation.
+#
+# Define ASCIIDOCTOR_EXTENSIONS_LAB to point to the location of the Asciidoctor
+# Extensions Lab if you have it available.
+#
 # Define PERL_PATH to the path of your Perl binary (usually /usr/bin/perl).
 #
 # Define NO_PERL_MAKEMAKER if you cannot use Makefiles generated by perl's