configure.ac: Check for unbound version >= 1.5

In order to do this and provide good feedback, a new macro was
created (AST_EXT_LIB_EXTRA_CHECK) which does the normal check and
path setups for the library then compiles, links and runs a supplied
code fragment to do the final determination.  In this case, the
final code fragment compares UNBOUND_VERSION_MAJOR
and UNBOUND_VERSION_MINOR to determine if they're greater than or
equal to 1.5.

Since we require version 1.5, some code in res_resolver_unbound
was also simplified.

ASTERISK-28045
Reported by: Samuel Galarneau

Change-Id: Iee94ad543cd6f8b118df8c4c7afd9c4e2ca1fa72