BUG: Fix link error related to ITKExpat when using ITK > v4.8.1

ITKEXPAT is no longer used and has been replaced by the ITKExpat module which
is now included by setting the project's COMPONENTS. This allows the use of a
system installed and packaged expat library, which saves build time and
improves integration into package managers.
See InsightSoftwareConsortium/ITK@ad26080

This commit integrates contribution by Hans Johnson (See PR #48), where
ITKExpat is add to the COMPONENTS to find_packages ITK.

This results in the following test failures:

          6 - GenerateCLPTest-Slicer3Example-Build (Failed)
          7 - GenerateCLPTest-Slicer3Example-Test (Failed)
          9 - GenerateCLPTest-PointExample1-Build (Failed)
         11 - GenerateCLPTest-CLPExample1-Build (Failed)

That fail with the error:

11:   [3/3] Linking CXX executable CLPExample1
11:
11:   FAILED: : && /home/matt/bin/exe/c++ -Os -DNDEBUG
11:   CMakeFiles/CLPExample1.dir/CLPExample1.cxx.o -o CLPExample1 -rdynamic
11:   -lITKEXPAT && :
11:
11:
11:   /usr/lib/gcc/x86_64-pc-linux-gnu/4.8.5/../../../../x86_64-pc-linux-gnu/bin/ld:
11:   cannot find -lITKEXPAT

Author: LucasGandel

GenerateCLP/Testing/CLPExample1/CMakeLists.txt

@@ -26,8 +26,9 @@ endif()
 set(${PROJECT_NAME}_SOURCE ${PROJECT_NAME}.cxx)
 GENERATECLP(${PROJECT_NAME}_SOURCE ${PROJECT_NAME}.xml)
 add_executable(${PROJECT_NAME} ${${PROJECT_NAME}_SOURCE})
-target_link_libraries(${PROJECT_NAME} ITKEXPAT ${_additional_link_libraries})
-
+if(_additional_link_libraries)
+  target_link_libraries(${PROJECT_NAME} ${_additional_link_libraries})
+endif()
 #-----------------------------------------------------------------------------
 # Test
 #-----------------------------------------------------------------------------

GenerateCLP/Testing/PointExample1/CMakeLists.txt

@@ -26,4 +26,6 @@ endif()
 set(${PROJECT_NAME}_SOURCE ${PROJECT_NAME}.cxx)
 GENERATECLP(${PROJECT_NAME}_SOURCE ${PROJECT_NAME}.xml)
 add_executable(${PROJECT_NAME} ${${PROJECT_NAME}_SOURCE})
-target_link_libraries(${PROJECT_NAME} ITKEXPAT ${_additional_link_libraries})
+if(_additional_link_libraries)
+  target_link_libraries(${PROJECT_NAME} ${_additional_link_libraries})
+endif()

GenerateCLP/Testing/Slicer3Example/CMakeLists.txt

@@ -26,7 +26,9 @@ endif()
 set(${PROJECT_NAME}_SOURCE ${PROJECT_NAME}.cxx)
 GENERATECLP(${PROJECT_NAME}_SOURCE ${PROJECT_NAME}.xml)
 add_executable(${PROJECT_NAME} ${${PROJECT_NAME}_SOURCE})
-target_link_libraries(${PROJECT_NAME} ITKEXPAT ${_additional_link_libraries})
+if(_additional_link_libraries)
+  target_link_libraries(${PROJECT_NAME} ${_additional_link_libraries})
+endif()
 
 #-----------------------------------------------------------------------------
 # Test