docs/manual: bump our gcc/g++ requirements to 4.8

docs/manual: bump our gcc/g++ requirements to 4.8

Currently, we only require a gcc 4.4 version, which now is pretty old
(released in April 2009).  This requirement is not even tested nowadays,
with our oldest autobuilder having a 4.7 version only.

And even then, 4.7 is still old enough that it prevents us from
upgrading some packages. For example cmake 3.10+ requires C++11
constructs that were only added in gcc 4.8 (when C++11 support was
finally completed in gcc).

So, update our requirements for gcc to at least 4.8.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>