Merge branch 'master' into core-updates

build: Have `pdf' depend on the PDF image.

* doc.am (DOT_OPTIONS): Remove `-Tpng'.
  (.dot.pdf): New rule.
  (pdf-local): Depend on the .pdf.

build: Use `missing' for `dot'.

* configure.ac: Substitute `DOT'.
* doc.am (.dot.png, .dot.eps): Use $(DOT).

build: Build docs from the top-level Makefile.

This undoes commit 575ed8d ("doc: Create own Makefile.am in subdirectory."),
and fixes a bug whereby "make pdf" or "make ps" wouldn't do anything.

* doc/Makefile.am: Remove.
* doc.am: New file.  Use `-local' rules to build the image.
* Makefile.am: Include it.
* configure.ac: Use `-Woverride' to avoid undesired overridding of
  Automake rules.

doc: Create own Makefile.am in subdirectory.

* doc/Makefile.am: New file.
* Makefile.am: Reference subdirectory doc.