In WT-8196, we defined AM_CXXFLAGS which contains the first set of compilation flags. In WT-8247, we should have a complete list of flags we want to keep. It's not time to enable them for the bench/workgen files.

In WT-8196, we are overwriting AM_CXXFLAGS to an empty list, hence no flags are taken into account. This ticket should remove that workaround and fix each warning is triggered by the flags defined in AM_CXXFLAGS.

Definition of done:

The cpp files in bench/workgen compile with the flags defined by AM_CXXFLAGS and no warnings are generated during the compilation.