port codebase to gnu++11. Since boost 1.83, it is necessary to build rsem with C++ 2011 standard. This patch bumps the -std=gnu++98 passed by upstream Makefil to -std=gnu++11. It should become unnecessary once upstream bumped to C++ 2011 or later, or when the package will not depend on boost anymore, in case this is in upstream's agenda; there is a merge request open upstream to bump to -std=gnu++17 in the meantime.
