In preparation for multilib support, use the same "backend" model that
is already employed by glibc and musl.
Also, the verbosity setting descriptions were swapped. V=2 is actually
less verbose than V=1: V=1 prints full commands, while V=2 prints 'CC
<file> <defines>'.
Signed-off-by: Alexey Neyman <stilor@att.net>