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="Gustavo Zacarias <gustavo@zacarias.com.ar>"

reporter_url="http://sourceware.org/ml/crossgcc/2011-03/msg00028.html"

reporter_comment=""