AC_CHECK_DECL, AC_CHECK_DECLS: port to the Clang compiler

* lib/autoconf/general.m4 (_AC_UNDECLARED_WARNING): New macro.
(_AC_CHECK_DECL_BODY): Call it once per language; treat warnings as
errors when its verdict indicates that.
* tests/semantics.at (AC_CHECK_DECLS): Add a macro call that relies on
the new semantics.  Avoid -Wmissing-variable-declarations warnings.
* doc/autoconf.texi (Generic Declarations): Document the implications.
* NEWS: Mention this change.