qpid-cpp: fix build after #28029

When -O2 from hardening does not redefine -O3 from CMake, the build fails with:

     src/qpid/broker/SelectorExpression.cpp: In member function ‘qpid::broker::Expression* qpid::broker::Parse::orExpression(qpid::broker::Tokeniser&)’:
     src/qpid/broker/SelectorExpression.cpp:1041:13: error: ‘*((void*)& s +17)’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
          if (s[1]=='b' || s[1]=='B') {