On Windows POCO include files un-define a few Windows macros by default, e.g. CreateMutex. While this avoids name clashes in third-party code which wants to define e.g. CreateMutex itself, it also leads to the opposite problem of third-party code not compiling anymore if it makes use of such macros.
For BlueBerry/MITK, we use the Windows macros so we prevent their un-definition by compiling our code with -DPOCO_NO_UNWINDOWS. For MITK modules and plug-ins, these compile defintions are already added by the respective macros. For applications, they are missing and may lead to compile errors depending on the set of included MITK header files.