Feature request: emit warnings for broken references

[anarthal]

When a \ref command is used and the target can't be found, I'd like to know, as this usually implies a mistake on my side. Doxygen emits such warnings.

[alandefreitas]

We had a warning, but the problem is it was warning too much because @ref some-excluded-or-external-symbol is also valid and should be converted to simple inline code or a link to a tagfile symbol. I guess we can restore that and make it an option.

[anarthal]

Hm, if there is a match in a tagfile, there should be no warning IMO. A link to an excluded symbol should be a warning I'd say - it's a broken link at the end of the day.