Don't search for vtkdiffs path, just you its target.

scripts/cmake/test/AddTest.cmake

@@ -97,7 +97,7 @@ function (AddTest)
 	if(AddTest_TESTER STREQUAL "numdiff" AND NOT NUMDIFF_TOOL_PATH)
 		return()
 	endif()
-	if(AddTest_TESTER STREQUAL "vtkdiff" AND NOT VTKDIFF_TOOL_PATH)
+	if(AddTest_TESTER STREQUAL "vtkdiff" AND NOT TARGET vtkdiff)
 		return()
 	endif()
 	if(AddTest_TESTER STREQUAL "memcheck" AND NOT GREP_TOOL_PATH)
@@ -115,7 +115,7 @@ function (AddTest)
 		set(SELECTED_DIFF_TOOL_PATH ${NUMDIFF_TOOL_PATH})
 		set(TESTER_ARGS "--statistics --absolute-tolerance=1e-5 --relative-tolerance=1e-4")
 	elseif(AddTest_TESTER STREQUAL "vtkdiff")
-		set(SELECTED_DIFF_TOOL_PATH ${VTKDIFF_TOOL_PATH})
+		set(SELECTED_DIFF_TOOL_PATH $<TARGET_FILE:vtkdiff>)
 		set(TESTER_ARGS "-q --abs 1e-5 --rel 1e-4")
 	endif()
 

scripts/cmake/test/Test.cmake

 # Find tools and data
 find_program(DIFF_TOOL_PATH diff)
 find_program(NUMDIFF_TOOL_PATH numdiff)
-find_program(VTKDIFF_TOOL_PATH vtkdiff ${CMAKE_BINARY_DIR}/bin)
 # find_program(TIME_TOOL_PATH time) # TODO: does not work Travis
 set(TIME_TOOL_PATH time)
 find_program(GREP_TOOL_PATH grep)