This is a basic implementation of an OpenEXR loader. This
"infrastructure" is required for any further work. It consists of:

* The build system changes.

* A C wrapper around the OpenEXR library, which is necessary as it's not
  possible to intermix GIMP's code with C++ code.

* A basic image loader. Chroma is not supported currently, and some
  other weird files like multi-view files are unsupported. These can be
  added when necessary. There is no UI, but it should be straightforward
  to add new features like this on top of this work.