This allows easily enabling the blocks clang extension.

* gtkdoc: Add 'check' kwarg

This runs gtkdoc-check in meson tests.

Also reorganize the gtkdoc test because we cannot reliably build
multiple doc into the same directory. Not all files generated by gtk-doc
are prefixed with the target name.

determine_rpath_dirs() can return paths to external dependencies not
in the build dir and passing them first as a link path leads to
g-ir-scanner for example linking against the already installed library
instead of the just built one.

This was reported in g-i: https://gitlab.gnome.org/GNOME/gobject-introspection/issues/272
and comes up quite often when a library adds some new symbols which aren't present in the
system library, which then makes linking fail.

The first place where the order is changed is _scan_gir_targets(), which looks like an unintentional
change in the refactoring in 8377ea45aa61efbe8e1a75b74

The second place in _get_link_args() has always been that way and only the rpath order is changed,
but it looks to me as if the same rules should apply here too.

"But surely nobody would indent by having spaces first,
and tabs after that. Right? Right?"

It turns out there's a bug in creating a sub dependency out of threads
in that we pass all of the kwargs from the parent to the
ThreadDependency instance. This demonstrates the bug.