gnatstub creates body stubs, that is, empty but compilable bodies
for library unit declarations.
To create a body stub,
gnatstub has to compile the library
unit declaration. Therefore, bodies can be created only for legal
library units. Moreover, if a library unit depends semantically upon
units located outside the current directory, you have to provide
the source search path when calling
gnatstub, see the description
gnatstub switches below.