dladdr requires link with the libdl library.
Prune libraries linked with the unittest program.
Add clear method to AtomicStructureAdapter.
Use AtomicStructureAdapter for the empty singleton.
Replace VR3Structure with AtomicStructureAdapter.
Remove VR3Structure and associated classes.

Not used anymore.
Avoid scons crash after source file removal.

Use GlobSources that returns only objects that exist in the source
directory.  SCons Glob can return zombie copies from the vartiant dirs.