Fixes "warning: --output and -o have no effect when -C or --ccode is set"
get_always_args() adds -C which is already disabling the direct compilation
ability of valac for which -o is used.
As the Vala compiler does not define thread_flags() and
thread_link_flags(), depending on threads in any capacity will cause Meson to
fail.
Fixes#2720.
Rico Tzschichholz