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="Alexander BIGGA"

reporter_url="http://sourceware.org/ml/crossgcc/2008-06/msg00031.html"

reporter_comment=""