Since Configure does the signal name/number probe with awk

Since Configure does the signal name/number probe with awk
based on the cpp results (not by really running code), testing
for the signum value is not really helpful if we have runtime
function calls (some signal numbers are dynamic, for example)
(or other extra stuff where the signum value is expected).
(Runtimeness also means that Configure cannot get these right,
anyway.) Better filter in just the constant non-negative numbers.

p4raw-id: //depot/perl@18973