Added new USES_TERMINAL option to the ExternalProject_Add_Step
function. This option passes USES_TERMINAL to the underlying
add_custom_command call so that the Ninja console pool is used.
Also, corresponding new USES_TERMINAL_<step> options were added
to the ExternalProject_Add function.
Justification: if using Ninja with a CMake superbuild, it's often
desirable to limit the superbuild to ONE sub-Ninja process at a
time to avoid oversubscribing the CPU. Using the console pool also
makes it easy to monitor the progress of the sub-Ninja process.
Independent USES_TERMINAL_<step> arguments are passed to
ExternalProject_Add instead of one USES_TERMINAL argument that
controls everything. Users may wish to run some steps in parallel
but not others (e.g. parallelize configure but not build).
Use file(GENERATE) to write the initial cache file so that we can
evaluate generator expressions. Use a per-config initial cache file
name in case the content varies by configuration.
James Johnston
Brad King
Andrey Pokrovskiy
Daniele E. Domenichelli