docs/manual: document gcc version dependencies

This commit updates the Buildroot manual to document how to detail the
gcc version dependencies in Config.in comments of packages, like we do
for kernel headers version.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>