docs: make the settings docs work from tarball builds

docs: make the settings docs work from tarball builds

docs/api/settings-spec.xml was accidentally not getting disted,
because gtk-doc.make explicitly removes all DISTCLEANFILES from
distdir. However, it doesn't actually make sense for the settings docs
files to be in DISTCLEANFILES anyway; they were put there rather than
CLEANFILES (IIRC) so that "make clean" in a tarball build wouldn't
delete them and break things. But the right fix is to just make them
only be in CLEANFILES when BUILD_SETTING_DOCS is true, and not ever
get deleted otherwise.

Also adjust the build rules to ensure that the generated docs don't
get rebuilt in tarball builds, since that can cause problems when
building from a read-only source tree, etc.

Meanwhile, in an unrelated but also fatal bug, configure.ac's check
for if the generated docs were already present never got updated for
the cli/src -> clients/cli move, and so even if we had been disting
settings-spec.xml, configure would still think that the tarball didn't
have all of the generated docs in it, so SETTING_DOCS_AVAILABLE would
be set false and none of the generated docs would get used.

https://bugzilla.gnome.org/show_bug.cgi?id=740035