Certain build configurations (like --enable-gtk-doc
--disable-introspection) were broken with respect to nm-setting-docs.
Fix this. Also, we don't require just gobject-introspection, we need
pygobject specifically as well.

Generate docs/api/settings-spec.xml via an XSLT stylesheet applied to
libnm-util/nm-setting-docs.xml.

dist tarballs shouldn't ever be released without the documentation,
and this allows us to build the tarballs with pre-generated docs
that get installed on the end system, but don't need to be built
there.  So the end system doesn't need gtk-doc installed, only
the dist system does.

https://bugzilla.gnome.org/show_bug.cgi?id=700093

"./autogen.sh --enable-doc && make" produced this error:
warning: failed to load external entity "../settings-spec.xml"
../network-manager-docs.xml:57: element include: XInclude error : could not load ../settings-spec.xml, and no fallback was found

Removing settings-spec.xml from $(content_files) made the file non-DISTed but it
also removed the file as a dependency for html-build.stamp that also runs
cd html && gtkdoc-mkhtml $$mkhtml_options $(MKHTML_OPTIONS) $(DOC_MODULE) ../$(DOC_MAIN_SGML_FILE)
and $(DOC_MAIN_SGML_FILE) includes settings-spec.xml.

Fix that by making $(DOC_MAIN_SGML_FILE) dependent on setting-spec.xml.

The generated settings-spec.xml should not be DIST-ed because it's
regenerated every time libnm-util changes.  That meants it gets
rebuilt during 'make' even from a dist tarball.  Thus, it shouldn't
be part of the dist tarball itself.

In addition, settings-spec.html shouldn't be included in
$(content_files) becuase this is added to EXTRA_DIST by gtk-doc.make.

Since generate-settings-spec.c the move from docs/api/ to tools/,
this started breaking distcheck even though the original Makefile
rules were wrong too.

The refentry xml is used to generate manual page with settings description.

The invocation is:
generate-settings-spec <type> <output file> [<type> <output file>]
where <type> is  "book" or "refentry"