ENH: use foamVersion::api internally in etcFiles searching (#1010)

- prefer this to using the OPENFOAM define since this improves the
  internal consistency with the build information.

  The API information could change between builds without the
  etcFiles.C being recompiled whereas the value of
  Foam::foamVersion::api is force updated during the build (triggers
  recompilation of globals.Cver)