[patch][pr17429] Do nothing in a null build if CLANG_ALWAYS_CHECK_VC_REV is OFF

[Rafael Espíndola]

On 1 October 2013 11:56, Jordan Rose <jordan_rose at apple.com> wrote:
> Ah. The idea is that the version at least gets updated when you reconfigure, but that's already there by removing the output file.
>
> Please don't remove HAVE_SVN_VERSION_INC, though. A slightly outdated version is still better than no version at all.

Cmake deletes the file (on reconfigure?) since it is marked as
generated. If the dependency to clang_revision_tag is not there and we
keep HAVE_SVN_VERSION_INC defined the build fails.

Cheers,
Rafael