First, how useful would be that indication whether a module gets PASS or FAIL ?

All a FAIL means is that the module failed to build on the particular tester's machine. That could mean there's something wrong with the module ... or it could also mean that there's something wrong with the tester's build environment.

I have some modules that rely on external C libraries - and I'm always getting FAIL reports from testers because they don't have the requisite C library installed, or because their version of the requisite C library is too old.

jettero mentioned Devel::CheckLib - and it can, under some circumstances, help out with that. But it entails adding extra cruft to the Makefile.PL, and I would prefer to *not* do that. On too many occasions I've had to work through a Makefile.PL, trying to fix code that has been introduced solely for the purpose of avoiding FAIL reports from testers. Often I've found that once that code has been removed, the module builds flawlessly for me. That's very frustrating - the author has put obstacles in my path out of the (misplaced) vanity of wanting to avoid "FAIL" reports from testers. That's a crime that I would like to avoid. (Not everyone shares my view on this :-)

Cheers,
Rob