Do not set G4VERBOSE to ON when building in Debug

Do not set G4VERBOSE to ON when building in Debug. This is to avoid GiB of log messages of the style:

G4WT0 > ### pop requested out of 5 stacked tracks.
G4WT0 > Selected G4StackedTrack : 0x7fce263898e0 with G4Track 0x7fce20a390d8 (trackID 679084, parentID 1)
G4WT0 > Track 0x7fce20a390d8 (trackID 679084, parentID 1) is passed to G4TrackingManager.
G4WT0 > Track (trackID 679084, parentID 1) is processed with stopping code 2
G4WT0 > A new track 0x7fce20a38fd0 (trackID 679091, parentID 1) is passed to G4StackManager.
G4WT0 > A new track 0x7fce20a393f0 (trackID 679092, parentID 1) is passed to G4StackManager.
G4WT0 > A new track 0x7fce20a394f8 (trackID 679093, parentID 1) is passed to G4StackManager.
G4WT0 > A new track 0x7fce20a39600 (trackID 679094, parentID 1) is passed to G4StackManager.
G4WT0 > A new track 0x7fce20a39ca0 (trackID 679095, parentID 1) is passed to G4StackManager.
G4WT0 > A new track 0x7fce20a391e0 (trackID 679096, parentID 1) is passed to G4StackManager.
G4WT0 > A new track 0x7fce20a39da8 (trackID 679097, parentID 1) is passed to G4StackManager.
G4WT0 > ### pop requested out of 11 stacked tracks.
G4WT0 > Selected G4StackedTrack : 0x7fce263898e0 with G4Track 0x7fce20a39da8 (trackID 679097, parentID 1)
G4WT0 > Track 0x7fce20a39da8 (trackID 679097, parentID 1) is passed to G4TrackingManager.
G4WT0 > Track (trackID 679097, parentID 1) is processed with stopping code 2

when running with the Debug build. @clemenci @gcorti

Closes #15 (closed)