[cmake-developers] Using USES_TERMINAL in ExternalProject.cmake

[Robert Dailey]

In my own personal usage of add_custom_target() and
add_custom_command(), I use USES_TERMINAL so that stdout and stderr
from commands that are actively running get output in more real-time.
I've only tested this using the Ninja generator, and seems to improve
output behavior.

Without it, I notice that commands that take a long time to complete
result in a large window of silence. When the command completes, all
of its output (sometimes thousands of lines) gets spit out all at
once. This makes things weird, especially for things like download
progress.

I noticed that ExternalProject_Add() has this same issue when run from
Ninja. I do not see real-time output of the progress. Would it work to
set USES_TERMINAL for most (if not all) custom commands and targets in
ExternalProject.cmake?