Imagemagick with OpenEXR support

[Théo Maxime Tyburn]

Hi guixers,

I am trying to add OpenEXR to the current Imagemagick.

I tried this out
```
(package
  (inherit imagemagick)
  (inputs (append `(("openexr" ,openexr)) (package-inputs imagemagick))))
```

but in the configure phase:
```
checking for OpenEXR >= 1.0.6... no
[...]
Delegate library configuration:
OpenEXR           --with-openexr=yes          no
```

I'm not sure why OpenEXR is not visible at configure time. There doesn't
seem to be any manual addition of references to include or lib
directories from the store in the current imagemagick package
definition. So I assume it should just work fine.

Any idea?

Théo