Doc: auto-detect changed build flags

If you build the documentation switching between different options,
e.g., to build with both Asciidoc and Asciidoctor, you'll probably find
yourself running `make -C Documentation clean` either too often (wasting
time) or too rarely (getting mixed builds).

Track the flags we're using in the documentation build, similar to how
the main Makefile tracks CFLAGS and prefix flags. Track ASCIIDOC_COMMON
directly rather than its individual components -- that should make it
harder to forget to update the tracking if/when we modify the build
commands.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/.gitignore

@@ -13,3 +13,4 @@ mergetools-*.txt
 manpage-base-url.xsl
 SubmittingPatches.txt
 tmp-doc-diff/
+GIT-ASCIIDOCFLAGS

Documentation/Makefile

@@ -331,6 +331,15 @@ mergetools-list.made: ../git-mergetool--lib.sh $(wildcard ../mergetools/*)
 		show_tool_names can_merge "* " || :' >mergetools-merge.txt && \
 	date >$@
 
+TRACK_ASCIIDOCFLAGS = $(subst ','\'',$(ASCIIDOC_COMMON):$(ASCIIDOC_HTML):$(ASCIIDOC_DOCBOOK))
+
+GIT-ASCIIDOCFLAGS: FORCE
+	@FLAGS='$(TRACK_ASCIIDOCFLAGS)'; \
+	    if test x"$$FLAGS" != x"`cat GIT-ASCIIDOCFLAGS 2>/dev/null`" ; then \
+		echo >&2 "    * new asciidoc flags"; \
+		echo "$$FLAGS" >GIT-ASCIIDOCFLAGS; \
+            fi
+
 clean:
 	$(RM) *.xml *.xml+ *.html *.html+ *.1 *.5 *.7
 	$(RM) *.texi *.texi+ *.texi++ git.info gitman.info
@@ -340,13 +349,14 @@ clean:
 	$(RM) SubmittingPatches.txt
 	$(RM) $(cmds_txt) $(mergetools_txt) *.made
 	$(RM) manpage-base-url.xsl
+	$(RM) GIT-ASCIIDOCFLAGS
 
-$(MAN_HTML): %.html : %.txt asciidoc.conf
+$(MAN_HTML): %.html : %.txt asciidoc.conf GIT-ASCIIDOCFLAGS
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
 	$(TXT_TO_HTML) -d manpage -o $@+ $< && \
 	mv $@+ $@
 
-$(OBSOLETE_HTML): %.html : %.txto asciidoc.conf
+$(OBSOLETE_HTML): %.html : %.txto asciidoc.conf GIT-ASCIIDOCFLAGS
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
 	$(TXT_TO_HTML) -o $@+ $< && \
 	mv $@+ $@
@@ -358,12 +368,12 @@ manpage-base-url.xsl: manpage-base-url.xsl.in
 	$(QUIET_XMLTO)$(RM) $@ && \
 	$(XMLTO) -m $(MANPAGE_XSL) $(XMLTO_EXTRA) man $<
 
-%.xml : %.txt asciidoc.conf
+%.xml : %.txt asciidoc.conf GIT-ASCIIDOCFLAGS
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
 	$(TXT_TO_XML) -d manpage -o $@+ $< && \
 	mv $@+ $@
 
-user-manual.xml: user-manual.txt user-manual.conf
+user-manual.xml: user-manual.txt user-manual.conf GIT-ASCIIDOCFLAGS
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
 	$(TXT_TO_XML) -d book -o $@+ $< && \
 	mv $@+ $@
@@ -373,7 +383,8 @@ technical/api-index.txt: technical/api-index-skel.txt \
 	$(QUIET_GEN)cd technical && '$(SHELL_PATH_SQ)' ./api-index.sh
 
 technical/%.html: ASCIIDOC_EXTRA += -a git-relative-html-prefix=../
-$(patsubst %,%.html,$(API_DOCS) technical/api-index $(TECH_DOCS)): %.html : %.txt asciidoc.conf
+$(patsubst %,%.html,$(API_DOCS) technical/api-index $(TECH_DOCS)): %.html : %.txt \
+	asciidoc.conf GIT-ASCIIDOCFLAGS
 	$(QUIET_ASCIIDOC)$(TXT_TO_HTML) $*.txt
 
 SubmittingPatches.txt: SubmittingPatches
@@ -430,7 +441,7 @@ $(patsubst %,%.html,$(ARTICLES)) : %.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
+$(patsubst %.txt,%.html,$(wildcard howto/*.txt)): %.html : %.txt GIT-ASCIIDOCFLAGS
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
 	sed -e '1,/^$$/d' $< | \
 	$(TXT_TO_HTML) - >$@+ && \