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 M68k-specific values

CT_DoArchTupleValues() {

    :

}