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.
A main scenario where this is needed is when a git operation
needs the password to a private key and asks for it on the
console. Without this change, such operations can appear to
hang indefinitely with no prompt if QUIET is in effect (which it
is by default).
Another scenario this addresses is when progress of a
download or update should be shown. Without this change,
all such progress is buffered with some generators and will
only be shown at the end, which defeats the purpose of logging
any progress to begin with.
Relates: #18238
