Conditional files and recompilation

Hello,

I have a project which use another one (sub-project). Both projects use 
a conditional file (-gnatep=...)(one conditional file per project).

I have noticed that each time I build a program from the main project, 
the sub-project is recompiled even if it has not changed (which slows 
down the compilation). If I build all the targets the sub-project is 
recompiled for each target. I never see the "target up-to date message".

Is it due to the use of conditional files or to another option?

Thanks in advance,
Mathieu