Several major Linux distributions still do not provide Git >= 1.6.5.0 in
their stable package lists.  Prior to commit 1173cc4a (Update Git
submodules with --recursive, 2011-02-22) CTest was able to use older Git
versions but simply silently failed to update submodules recursively.
Instead of failing with older Git versions preserve the status quo and
add a warning in the update log.  Users testing projects with recursive
submodules may simply update to a Git new enough to support them.