infinity and NaN are not supported by all perls

[patch]

Some older perls on some systems don’t support inf and nan. Here are a few failing test reports from CLDR::Number v0.12.

• perl=5.12.2 os=mswin32
• perl=5.10.1 os=mswin32
• perl=5.8.7 os=darwin
• perl=5.8.7 os=darwin

I think we should just test for support in the test file t/inf-nan.t and skip with a diag warning when not supported, as well as documenting that the feature depends on perl’s support for the given system.