cmake: set CMP0135 policy

cmake: set CMP0135 policy

so the `DOWNLOAD_EXTRACT_TIMESTAMP` property of
`ExternalProject_Add()` command is set by default on CMake v3.24 and up.
it helps to set the a more accurate timestamp for the downloaded
content, hence the targets depending on the extracted content can be
rebuilt if the URL changes.

see also https://cmake.org/cmake/help/latest/policy/CMP0135.html

Signed-off-by: Kefu Chai <tchaikov@gmail.com>