package/libabseil-cpp: depend on (host) gcc >= 4.9

abseil depends on gcc >= 4.9 according to [0] and actually doesn't
compile anymore with the latest version bump.
This didn't show in the autobuilders as the C++11 issue fixed in the
previous commit shadowed it.

also update dependency in package/grpc.

fixes:
 https://github.com/abseil/abseil-cpp/issues/795

[0] https://abseil.io/docs/cpp/platforms/platforms#linux



Signed-off-by: Michael Nosthoff <buildroot@heine.tech>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>