build: Fix Makefile of valtest and opaquetest samples

Some parts were not taking into account the fact that the generated code
is now in a subfolder.
gtk: Add ListStore.SetValue overload for long type

Without that change, using ListStore.SetValue with a long would use the
float overload, which might not be expected and cause some issues.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>