Fix configure with clang avoiding nested function

The detection of those issues was made by openbsd people (ajacoutot and rnagy)
See http://www.undeadly.org/cgi?action=article;sid=20170930133438

For this port our fix is slightly different than what they did, still the idea
is the same

Reported by: ajacoutot@openbsd, rnagy@openbsd