Introduces `BUILD_JOB_SERVER_AWARE` option to `ExternalProject_Add` and
`JOB_SERVER_AWARE` to `ExternalProject_Add_Step`. When using an explicit
`BUILD_COMMAND` or `COMMAND`, the generated commands won't use `$(MAKE)`
thus failing to connect to the outer make's job server. These new
options enable explicit job server integration.
Co-authored-by: Brad King <brad.king@kitware.com>
Fixes: #16273
