r22772: - Still working on the fact that readahead() is not declared (on at least one

r22772: - Still working on the fact that readahead() is not declared (on at least one
  OS) but is available for linking.  Instead of running configure tests with
  -Werror-implicit-function-declaration in developer mode (which may lead to
  different library functions being used in developer mode than when not in
  developer mode), add tests for whether readahead is declared.  If not,
  provide a replacement declaration in lib/replace.
(This used to be commit 7d05fa8b32b5b33e95fd9d552d2a45013b4803ec)