Fix for DEBUG pre-processor related issues in Xcode

Fixes: DEBUG pre-processor causing Symbolic errors when OpenEXR is
compiled for Static Release and then linked against with a project
built in Debug Targeted Xcode Project.

Renames: ```DEBUG``` to OPENEXR_DEBUG
Added pre-processor ```OPENEXR_DEBUG``` in place of ```_DEBUG``` to
debug build of OpenEXR for debugging purposes.