[CMake] Fix a dependency that was ignored.

The intention of the __allBuiltins was to build buildins as part of
moving headers. However, CMake ignores a target dependency if it's not
added with add_dependencies().

Author: hageboeck

CMakeLists.txt

@@ -384,8 +384,13 @@ ROOT_ADD_TEST_SUBDIRECTORY(tutorials)
 ROOT_CREATE_HEADER_COPY_TARGETS()
 
 get_property(__allHeaders GLOBAL PROPERTY ROOT_HEADER_TARGETS)
+add_custom_target(move_headers DEPENDS ${__allHeaders} gitinfotxt)
+
 get_property(__allBuiltins GLOBAL PROPERTY ROOT_BUILTIN_TARGETS)
-add_custom_target(move_headers ALL DEPENDS ${__allHeaders} ${__allBuiltins} gitinfotxt)
+list(LENGTH __allBuiltins num_builtins)
+if(${num_builtins} GREATER 0)
+  add_dependencies(move_headers ${__allBuiltins})
+endif()
 
 #---CXX MODULES-----------------------------------------------------------------------------------
 if(MSVC)