If an attempt is made to use the Autoconf editor on an external configure.in
file, the editor causes a ClassCastException to occur. The editor should allow
external files and mark them appropriately.
The logic was using the IResource marker utilities for error handling (e.g.
warnings and error markers) for the file. These utilities only work for
resources in the workspace (i.e. files in projects in the workspace). A patch
has been made to the HEAD repository so the error handler for the Autoconf
editor performs marking manually and does not require an IResource.