maint: suppress bogus noreturn warnings

maint: suppress bogus noreturn warnings

* configure.ac: Move the single-binary code before the
gcc-warnings code, so that the latter can depend on the former.
Suppress -Wsuggest-attribute=noreturn with single binaries,
to avoid diagnostics like the following:
  src/expr.c: In function 'single_binary_main_expr':
  error: function might be candidate for attribute 'noreturn'
Problem reported by Pádraig Brady in:
https://lists.gnu.org/r/coreutils/2022-01/msg00061.html