Build target implementators might often like to
check that the actual graph of dependencies does
not contain cycles in order to report an informed
message before the build system does that on
compilation. 

For example, in the project for a library L ,
using a library that uses L is illegal. The error
should be reported directly in the dialog for
choosing libraries (or even better, the library
should be disabled) rather than allowing to add
that library and failing on compilation. 

Each module could code this by hand, but as the
projects have to contain that functionality
anyway, it seems beneficial to me to expose it in
a utility function for other modules to check the
correctness of dependencies at will.