ENH: Add custom target names "CTK" to superbuild.

That target allows to build CTK project itself directly from the superbuild
directory. For example, on linux, it could be done using "make CTK"

SuperBuild.cmake

@@ -313,7 +313,7 @@ ExternalProject_Add(${proj}
 #-----------------------------------------------------------------------------
 # CTK
 #
-set(proj CTK-build)
+SET(proj CTK-build)
 ExternalProject_Add(${proj}
   DOWNLOAD_COMMAND ""
   CMAKE_GENERATOR ${gen}
@@ -324,3 +324,12 @@ ExternalProject_Add(${proj}
   DEPENDS
     "CTK-Configure"
   )
+
+#-----------------------------------------------------------------------------
+# Custom target allowing to drive the build of CTK project itself
+#
+ADD_CUSTOM_TARGET(CTK
+  COMMAND ${CMAKE_COMMAND} --build ${CMAKE_CURRENT_BINARY_DIR}/CTK-build
+  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/CTK-build
+  )
+