fix gcc warnings

Common/Options.cpp

Common/StringUtils.cpp

Fltk/Solvers.cpp

Fltk/classificationEditor.cpp

Fltk/fieldWindow.cpp

Fltk/pluginWindow.cpp

Fltk/projectionEditor.cpp

Fltk/solverWindow.cpp

Fltk/visibilityWindow.cpp

Geo/GEdge.cpp

Geo/GEdgeCompound.cpp

Geo/GEntity.cpp

Geo/GModel.cpp

Geo/GModelIO_Fourier.h

Geo/Geo.cpp

Geo/GeomMeshMatcher.cpp

Geo/MElementCut.h

Geo/MTetrahedron.cpp

Geo/OCC_Connect.cpp

Geo/SOrientedBoundingBox.cpp