Documentation: move some AsciiDoc parameters into variables

Asciidoctor takes slightly different arguments from AsciiDoc in some
cases.  It has a different name for the HTML backend and the "docbook"
backend produces DocBook 5, not DocBook 4.5.  Also, Asciidoctor does not
accept the -f option.  Move these values into variables so that they can
be overridden by users wishing to use Asciidoctor instead of Asciidoc.

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

Documentation/Makefile

@@ -97,6 +97,9 @@ man7dir = $(mandir)/man7
 
 ASCIIDOC = asciidoc
 ASCIIDOC_EXTRA =
+ASCIIDOC_HTML = xhtml11
+ASCIIDOC_DOCBOOK = docbook
+ASCIIDOC_CONF = -f asciidoc.conf
 MANPAGE_XSL = manpage-normal.xsl
 XMLTO = xmlto
 XMLTO_EXTRA =
@@ -304,13 +307,13 @@ clean:
 
 $(MAN_HTML): %.html : %.txt asciidoc.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) -b xhtml11 -d manpage -f asciidoc.conf \
+	$(ASCIIDOC) -b $(ASCIIDOC_HTML) -d manpage $(ASCIIDOC_CONF) \
 		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) -o $@+ $< && \
 	mv $@+ $@
 
 $(OBSOLETE_HTML): %.html : %.txto asciidoc.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) -b xhtml11 -f asciidoc.conf \
+	$(ASCIIDOC) -b $(ASCIIDOC_HTML) $(ASCIIDOC_CONF) \
 		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) -o $@+ $< && \
 	mv $@+ $@
 
@@ -323,13 +326,13 @@ manpage-base-url.xsl: manpage-base-url.xsl.in
 
 %.xml : %.txt asciidoc.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) -b docbook -d manpage -f asciidoc.conf \
+	$(ASCIIDOC) -b $(ASCIIDOC_DOCBOOK) -d manpage $(ASCIIDOC_CONF) \
 		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) -o $@+ $< && \
 	mv $@+ $@
 
 user-manual.xml: user-manual.txt user-manual.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b docbook -d article -o $@+ $< && \
+	$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b $(ASCIIDOC_DOCBOOK) -d article -o $@+ $< && \
 	mv $@+ $@
 
 technical/api-index.txt: technical/api-index-skel.txt \
@@ -338,7 +341,7 @@ technical/api-index.txt: technical/api-index-skel.txt \
 
 technical/%.html: ASCIIDOC_EXTRA += -a git-relative-html-prefix=../
 $(patsubst %,%.html,$(API_DOCS) technical/api-index $(TECH_DOCS)): %.html : %.txt asciidoc.conf
-	$(QUIET_ASCIIDOC)$(ASCIIDOC) -b xhtml11 -f asciidoc.conf \
+	$(QUIET_ASCIIDOC)$(ASCIIDOC) -b $(ASCIIDOC_HTML) $(ASCIIDOC_CONF) \
 		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) $*.txt
 
 XSLT = docbook.xsl
@@ -386,14 +389,15 @@ howto-index.txt: howto-index.sh $(wildcard howto/*.txt)
 	mv $@+ $@
 
 $(patsubst %,%.html,$(ARTICLES)) : %.html : %.txt
-	$(QUIET_ASCIIDOC)$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b xhtml11 $*.txt
+	$(QUIET_ASCIIDOC)$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b $(ASCIIDOC_HTML) $*.txt
 
 WEBDOC_DEST = /pub/software/scm/git/docs
 
 howto/%.html: ASCIIDOC_EXTRA += -a git-relative-html-prefix=../
 $(patsubst %.txt,%.html,$(wildcard howto/*.txt)): %.html : %.txt
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	sed -e '1,/^$$/d' $< | $(ASCIIDOC) $(ASCIIDOC_EXTRA) -b xhtml11 - >$@+ && \
+	sed -e '1,/^$$/d' $< | \
+	$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b $(ASCIIDOC_HTML) - >$@+ && \
 	mv $@+ $@
 
 install-webdoc : html