BuildSystem: Fix misdetection of gethostbyname_r() on NetBSD

Fix the configure script not to detect the presence of gethostbyname_r()
on NetBSD incorrectly.  NetBSD includes it as an internal libc symbol
that is not exposed in system headers and that is incompatible with
other implementations.  In order to avoid misdetecting it, perform
the symbol check only if the declaration is found in the public header
first.

ASTERISK-29817

Change-Id: Iafa359b09908251bcd299ff54be003ea129b9eda