ExternalProject: Enable make job server with explicit build command
When an explicit BUILD_COMMAND ...
is used with Makefile generators,
the generated build command will not use $(MAKE)
so the nested make
process does not connect to the outer make's job server. Add a new
BUILD_JOB_SERVER_AWARE
option to enable the job server explicitly.
Fixes: #16273 (closed)
Co-authored-by: @brad.king
Edited by Brad King