package/stella: needs gcc >= 6

stella converted most of its C-Style arrays to std::array since version
6.1:
https://github.com/stella-emu/stella/commit/0c0f732e5f50b08cfcbdc5ad96c4e18ad230c04b

However, gcc 5.x does not accept a = {} initialization for std::array:

In file included from src/emucore/ConsoleIO.hxx:18:0,
                 from src/emucore/Console.hxx:34,
                 from src/gui/AudioDialog.cxx:22:
src/emucore/Control.hxx:331:71: error: array must be initialized with a brace-enclosed initializer
     std::array<bool, 5> myDigitalPinState{true, true, true, true, true};
                                                                       ^

So add a dependency on gcc >= 6 instead of trying to patch the numerous
array initializations to make them compliant with C++11

Fixes:
 - http://autobuild.buildroot.org/results/dfd9b901fabf310ed9033b8a012466c565d58684



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>