lib: Makefile.am: Fix "make distcheck" errors.
plugins: Fix "make distcheck" errors.
main: Makefile.am: Fix "make distcheck" errors.
doc: Revamp build system to comply with autotool defaults.

The doc was strange with the build done in special targets,
not invoked on ordinary "make all". As a  consequence, simple
"make distcheck" fails in a number of more or less interesting
ways.

New Makefile declares the docs as standard DATA variables handled
by automake and built by 'make all'.  This one actually passes
"make distcheck". The downside is that Makefile.am is full of
long lists.
README.md (github README): update.
Merge branch 'distcheck': make build system a little more sane.

Bigger change sin doc, which did not build or install the documents
in the expected way using 'make all'. "make distcheck" ran into all
sorts of more or less interesting errors.  New version declares
all docs using standard DATA variables handled by automake.

The rest is simple bugfixes. "make distcheck" now passes.