build: Turn dot into a "proper" maintainer program

The --enable-maintainer-mode option has been used in autogen.des for a
while now and it has allowed automatic reconfiguration and hot reload of
makefiles.

For the next step, we can turn certain programs into maintainer programs
and only require them when they are absolutely needed. This is what we
currently do with SVG files generated from dot files, checked in git and
optionally required, with an error message when rebuild is not possible.

It turns out we only need 3 lines in configure.ac to implement all that,
with the following differences:

- no more `./configure --with-dot[=...]` option
  - use `./configure DOT=...` to override the default `dot` command
  - or alternatively run configure with `DOT=...` in the environment
- generic missing error message (but referencing dot) from make
- no need to reconfigure once `dot` is in the PATH

The DOT override is actually documented in `./configure --help` like the
`--with-dot[=...]` option was.

While dot is our most emblematic maintainer tool, with its build targets
checked in all the way in the git repository, this could be applied to
other maintainer programs like sphinx-build and rst2man that should not
be required when building from a dist archive.

Makefile.am

@@ -85,19 +85,8 @@ witness.dot: all
 		lib/libvmod_*/
 
 .dot.svg:
-if ! HAVE_DOT
-	@echo ==================================================
-	@echo You need graphviz installed to generate svg output
-	@echo ==================================================
-	@false
-else
 	$(AM_V_GEN) $(DOT) -Tsvg $< >$@
-endif
 
-if HAVE_DOT
 witness: witness.svg
-else
-witness: witness.dot
-endif
 
 .PHONY: cscope witness.dot

configure.ac

@@ -79,16 +79,9 @@ if test "x$RST2HTML" = "xno"; then
      [rst2html not found - (Weird, we found rst2man?!)])
 fi
 
-AC_ARG_WITH([dot],
-  AS_HELP_STRING([--with-dot=PATH],
-		 [Location of the dot tool from graphviz (auto)]),
-  [DOT="$withval"],
-  [AC_CHECK_PROGS(DOT, [dot], [no])
-   if test "x$DOT" = "xno"; then
-     AC_MSG_WARN(
-       [dot not found - build will fail if svg files are out of date.])
-   fi])
-AM_CONDITIONAL(HAVE_DOT,[test "x$DOT" != "xno"])
+AC_ARG_VAR([DOT], [The dot program from graphviz to build SVG graphics])
+AM_MISSING_PROG([DOT], [dot])
+AC_CHECK_PROGS([DOT], [dot])
 
 # Define VMOD flags
 _VARNISH_VMOD_LDFLAGS

doc/graphviz/Makefile.am

@@ -35,10 +35,8 @@ SVGS = \
 	cache_req_fsm.svg \
 	cache_fetch.svg
 
-if HAVE_DOT
 CLEANFILES = \
 	$(PDFS)
-endif
 
 pdf: $(PDFS)
 
@@ -46,31 +44,10 @@ html: $(SVGS) link_srcdir
 
 # XXX does not fit onto a4 unless in landscape
 cache_fetch.pdf: cache_fetch.dot
-if ! HAVE_DOT
-	@echo ==================================================
-	@echo You need graphviz installed to generate pdf output
-	@echo ==================================================
-	@false
-else
-	@DOT@ -Tpdf -Gsize=$(SIZE) -Grotate=90 $< >$@
-endif
+	$(AM_V_GEN) $(DOT) -Tpdf -Gsize=$(SIZE) -Grotate=90 $< >$@
 
 .dot.pdf:
-if ! HAVE_DOT
-	@echo ==================================================
-	@echo You need graphviz installed to generate pdf output
-	@echo ==================================================
-	@false
-else
-	@DOT@ -Tpdf -Gsize=$(SIZE) $< >$@
-endif
+	$(AM_V_GEN) $(DOT) -Tpdf -Gsize=$(SIZE) $< >$@
 
 .dot.svg:
-if ! HAVE_DOT
-	@echo ==================================================
-	@echo You need graphviz installed to generate svg output
-	@echo ==================================================
-	@false
-else
-	@DOT@ -Tsvg $< >$@
-endif
+	$(AM_V_GEN) $(DOT) -Tsvg $< >$@