Fixing AC_CHECK_DECLS to do the right thing with clang

We've seen repeated complaints about bogus build warnings when using
"clang": it complains that strlcpy and some related library functions
haven't been declared. Several of the buildfarm animals exhibit such
warnings, for instance. That's because Autoconf's AC_CHECK_DECLS macro
fails to cope with the fact that clang only generates a warning, not
an error, for the test case that that macro uses. Noah fixed this
in upstream autoconf several years ago:

http://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=82ef7805faffa151e724aa76c245ec590d174580

However, I'm beginning to despair of the Autoconf crowd ever putting
out an official new release. Hence, I propose to apply and back-patch
the attached, which essentially just imports Noah's fix into our
configure script. I've verified that this does the right thing with
Fedora 28's version of clang (clang version 6.0.1).

regards, tom lane