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="Thomas Petazzoni"

 reporter_url="http://www.free-electrons.com/"

 reporter_comment="Very basic testing only"