Fix CMake CMP0135 warnings for ExternalProject downloads

[mvanhorn]

Sets cmake_policy(SET CMP0135 NEW) in lib/CMakeLists.txt so that
ExternalProject_Add URL downloads use extraction-time timestamps instead
of archive timestamps. This silences both dev warnings from the gbenchmark
and googletest downloads reported in #4852.

The NEW behavior is the recommended default since CMake 3.24 and ensures
that code depending on extracted contents is correctly rebuilt when the URL
changes.

Fixes #4852

[SeanTAllen]

Thanks for the PR!

The cmake_policy(SET CMP0135 NEW) approach has a compatibility problem: CMP0135 was introduced in CMake 3.24, but our minimum is 3.21. Calling cmake_policy(SET CMP0135 NEW) on CMake versions that don't know about that policy would be an error, so we'd have to bump the minimum to 3.24 to make this work.

Since the fix only applies to the two ExternalProject_Add calls in lib/CMakeLists.txt (gbenchmark on line 34 and googletest on line 41), I'd rather just add DOWNLOAD_EXTRACT_TIMESTAMP TRUE to each of them. That silences the warning without forcing a minimum version bump.

[mvanhorn]

Good catch on the minimum version. Switched to DOWNLOAD_EXTRACT_TIMESTAMP TRUE on both ExternalProject_Add calls in 2ccbf0e.

[SeanTAllen]

And I just noticed that DOWNLOAD_EXTRACT_TIMESTAMP is also Cmake 3.24. Ugh.

Some more searching suggests that something like

  if(POLICY CMP0135)
      cmake_policy(SET CMP0135 NEW)
  endif()

at the top of the file is "idiomatic" for handling this. but i haven't validated that.

also i might have gotten my reading of DOWNLOAD_EXTRACT_TIMESTAMP wrong and now I think maybe it would have been FALSE not TRUE. that we would have wanted. Not that it was the right thing but looks like I double-wronged it.

[mvanhorn]

Switched to the if(POLICY CMP0135) guard in 7bf880d. Removed the per-target DOWNLOAD_EXTRACT_TIMESTAMP since the policy applies globally to both ExternalProject_Add calls.