complibs: introduce a two-stage select

Directly select-ing the required companion libraries means we can not
disable them. That's OK for now, as we systematically build them when
they are required.

But with distros coming up-to-speed, we will need to disable the build
later-on.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Ioannis E. VENETIS"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

 reporter_comment=""