cc/gcc: do not build manuals in parallel

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Reported-by: Johannes Stezenbach <js@sig21.net>
Tested-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
(transplanted from c800b7d6a50c267c3e0a64829da4b082e260deff)

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }