Task #8161: fix build on CentOS 7. Add variants.dop256 to build on CentOS 7 (my laptop atm). Equalize compiler variants file options. Add CMAKE_SHARED_LINKER_FLAGS to set linker flags for shared libs (CentOS 7 GCC ld keeps default on not opting out overlinking, but can be changed now as in my variants file). Patch out build breakage when using log4cxx .