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).