Cycle detection too strong

[ghost]

If I specify something like:

lib c : c.wd : : : <implicit-dependency>c ;

I get an error about cycle in target references. This clearly not the case, since by the time <implicit-dependency>c is processed, the 'c' target is completely built.

The use case for this is when some of the headers exported by 'c' includes generated header. Workaround exists (with 'alias' rule), but better fix.