Fix configuration of CTKConfig based on DCMTK_HAVE_CONFIG_H_OPTIONAL value

This fixes the logic introduced by commit a32f524. Since the variable
is set to TRUE or FALSE only when DCMTK is looked up without using
DCMTKConfig, in case CTK is build against a version of DCMTK providing
DCMTKConfig, we need to also check if it is defined.

See #413

CMake/LastConfigureStep/CTKGenerateCTKConfig.cmake

@@ -162,7 +162,7 @@ set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}# External project libraries\n")
 set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}set(CTK_EXTERNAL_LIBRARIES \"${CTK_EXTERNAL_LIBRARIES}\")\n")
 set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}# External project library directories\n")
 set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}set(CTK_EXTERNAL_LIBRARY_DIRS \"${CTK_EXTERNAL_LIBRARY_DIRS}\")\n")
-if(NOT DCMTK_HAVE_CONFIG_H_OPTIONAL AND TARGET CTKDICOMCore)
+if(DEFINED DCMTK_HAVE_CONFIG_H_OPTIONAL AND NOT DCMTK_HAVE_CONFIG_H_OPTIONAL AND TARGET CTKDICOMCore)
   set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}# Definition required to build DCMTK dependent libraries\n")
   set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}if(\"\${CMAKE_VERSION}\" VERSION_GREATER 2.8.10)\n")
   set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}  set_target_properties(CTKDICOMCore PROPERTIES INTERFACE_COMPILE_DEFINITIONS ${DCMTK_DEFINITIONS})\n")
@@ -206,7 +206,7 @@ set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}# CTK library directories that could be u
 foreach(lib ${CTK_LIBRARIES})
   set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}set(${lib}_LIBRARY_DIRS \"\")\n")
 endforeach()
-if(NOT DCMTK_HAVE_CONFIG_H_OPTIONAL AND TARGET CTKDICOMCore)
+if(DEFINED DCMTK_HAVE_CONFIG_H_OPTIONAL AND NOT DCMTK_HAVE_CONFIG_H_OPTIONAL AND TARGET CTKDICOMCore)
   set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}# Definition required to build DCMTK dependent libraries\n")
   set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}if(\"\${CMAKE_VERSION}\" VERSION_GREATER 2.8.10)\n")
   set(CTK_CONFIG_CODE "${CTK_CONFIG_CODE}  set_target_properties(CTKDICOMCore PROPERTIES INTERFACE_COMPILE_DEFINITIONS ${DCMTK_DEFINITIONS})\n")