Ticket #757: CMake configuration: avoid compilation to fail with GCC < 6
-Wduplicated-cond and -Wnull-dereference are not supported with GCC version < 6, make them optional with GCC < 6
-Wduplicated-cond and -Wnull-dereference are not supported with GCC version < 6, make them optional with GCC < 6