[interpreter] Set CMake properties only if target available.

When the unittests were disabled temporarily, the condition still
evaluated to true, although the target didn't exist.

Author: hageboeck

interpreter/CMakeLists.txt

@@ -557,6 +557,6 @@ else()
 endif()
 
 add_subdirectory(CppInterOp)
-if(testing)
+if(TARGET CppInterOpUnitTests)
   set_target_properties(CppInterOpUnitTests PROPERTIES EXCLUDE_FROM_ALL OFF)
 endif()