Add the option to keep the current filestamps when extracting an
archive in ExternalProject_Add.
Enabling this option makes the behavior consistent with how
ExternalProject_Add is used when checking out code from revision
control instead of an archive.
Fixes: #22746
The ExternalProject module cannot be implemented in the Xcode "new build
system" without using CMP0114's NEW behavior. When configuring for that
build system, warn if the policy is not set to NEW and use NEW behavior
anyway.
