cc/gcc: comonalise the manuals build decision

Let the final frontend decide whether or not to build the manuals.

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=""