Fix compile warnings on missing override declarations
Explicit declare Q_DECL_OVERRIDE to silence compiler warnings.
Added -Wsuggest-override to CMAKE_CXX_FLAGS when the compiler is GCC
(clang already has this flag).
Differential Revision: https://phabricator.kde.org/D15400
(cherry picked from commit b6cbaf17dd2e0921c777a81a0637998b478e8321)