Opt-in skip broken symlinks check packaging

[lasote]

We need to package directories with broken symlinks. We are trying to package yocto sdk and we didn't manage to fix them or remove them, because the sdk stops working. So we would need something like CONAN_SKIP_BROKEN_SYMLINKS_CHECK to remove the check in manifest.py line 39

[DavidZemon]

Thank you for implementing this. This is something that forced me to spend most of a day "fixing" an otherwise perfectly fine package because I couldn't conanize it. I'm sure it isn't the only package we have that uses absolute symlinks, so knowing this feature exists should save me some time in the future.