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.
