This undoes commitgn-latest-20200428
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.