Make sure that WITH_OPENEXR is defined when sources are configured

with OpenEXR in the CMake system. This is a compilation fix from
Miguel A. Figueroa Villanueva for building Debug versions of Blender
using OpenEXR (thanks!).

source/blender/imbuf/intern/openexr/CMakeLists.txt

@@ -38,5 +38,9 @@ SET(INC
   ${OPENEXR_INC}
 )
 
+IF(WITH_OPENEXR)
+  ADD_DEFINITIONS(-DWITH_OPENEXR)
+ENDIF(WITH_OPENEXR)
+
 BLENDERLIB(bf_openexr "${SRC}" "${INC}")                 
 #env.BlenderLib ('bf_openexr', source_files, incs, defs, libtype=['core','player'], priority = [90, 200])