Fix for DEBUG pre-processor related issues in Xcode

###  Issue:
In Xcode when building a Debug targeted project the IDE adds ```-DEBUG``` to the target. This is causing many errors for libraries that use OpenEXR in it's release form and include headers which expose these parts of OpenEXR when that ```-DEBUG``` command is turned on (default action).

### Proposed change:
Swap out : ```DEBUG``` to ```OPENEXR_DEBUG```