dawgdic: enable with gcc 4.6

According to Nicolas Cavallari and confirmed by testing, std::strtoll
is indeed available in gcc 4.6, so the dawgdic package can be enabled
on gcc >= 4.6 instead of gcc >= 4.7.

Reported-by: Nicolas Cavallari <Nicolas.Cavallari@green-communications.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>