Support altenative include form in texi document extractor

@include requires the includee to follow certain format,
so we support "@c xinclude" alternative include form.
Emacs texinfo-multiple-files-update ignores it, but 'extract'
script inserts the included file in place.
Doc fix
Kludge to fix distclean target under libatomic_ops

Somehow automake dependency generation under libatomic_ops gets
screwed.  It generates "include" directive to .Plo files under
.deps, and it works for normal build, but when cleaning
the files get deleted earlier and make bails out by error.
I didn't figure out the exact cause; this patch just let
autoconf emit "-include" instead of "include", so that make
won't complain if the .Plo files are missing.
Generate files that need to reflect supported srfis from one source

The cond-feature list of srfis, lib/srfi/N.scm dummy modules,
and documents in concept.texi, are now generated from a single
source, src/srfis.scm.
Fix test failure in out-of-tree build