Previously cross, but not native, external args were used. Then in
d451a4bd97 the cross special cases were
removed, so external args are never used.
This commit switches that so they are always used. Sanity checking works
just the same as compiler checks like has header / has library.
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.
John Ericson