package/libcec: needs gcc >= 4.7

As per upstream, gcc-4.7 is required to build libcec:
    https://github.com/Pulse-Eight/libcec/issues/78#issuecomment-91578591

Fixes:
    http://autobuild.buildroot.org/results/3b5/3b5611725c13668472482ed4ad3b46886d4c63d9/
    http://autobuild.buildroot.org/results/0d0/0d032cdd8dbf6d0ddef392c3e5a734881e00ee64/
    http://autobuild.buildroot.org/results/c21/c21c555c38261f6911349ff9fdac6edeb4debaac/
    [...]

Possibly fixes:
    http://autobuild.buildroot.org/results/4a7/4a7b2af2fc3924255ac89e18f4f5c90bb8383265/
    http://autobuild.buildroot.org/results/dd6/dd61c186e6d85a9b30cd32edb74a52f7a4f73cc8/


    [...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>