package/libgpgme: C++ binding requires gcc 5

libgpgme now use the -Wsuggest-override warning parameter when building
the C++ binding. This requires gcc 5 or newer.

Fixes:
http://autobuild.buildroot.net/results/6eed9cae57af445f53c9854e007815b882c5c65a/
http://autobuild.buildroot.net/results/75617aee53a6f2b009a0d4ddc5b9a8c4cd406895/
http://autobuild.buildroot.net/results/8191a732918c6987b84309df3dfd66ab54b43fb7/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>