Add VDT to list of builtin targets so it builds before ROOT

The dependency of Vdt::Vdt imported target should have been
automatically added by CMake due to it being listed in the
BUILD_BYPRODUCTS of the ExternalProject_Add() command, but
not all versions of CMake work, so it is necessary to build
it early by force. Targets listed in ROOT_BUILTIN_TARGETS
get added as a dependency of the move_headers target, which
is reasonable since they often provide headers without which
ROOT cannot be built in any case (e.g. vdt/vdtMath.h).

cmake/modules/SearchInstalledSoftware.cmake

@@ -1471,7 +1471,8 @@ if(vdt OR builtin_vdt)
             DESTINATION ${CMAKE_INSTALL_LIBDIR} COMPONENT libraries)
     install(DIRECTORY ${CMAKE_BINARY_DIR}/include/vdt
             DESTINATION ${CMAKE_INSTALL_INCLUDEDIR} COMPONENT extra-headers)
-          set(vdt ON CACHE BOOL "Enabled because builtin_vdt enabled (${vdt_description})" FORCE)
+    set(vdt ON CACHE BOOL "Enabled because builtin_vdt enabled (${vdt_description})" FORCE)
+    set_property(GLOBAL APPEND PROPERTY ROOT_BUILTIN_TARGETS VDT)
   endif()
 endif()