Treat all targets serially at the toplevel. This allows

Treat all targets serially at the toplevel. This allows
to specify
  nice gmake --jobs all check
and go to lunch, while a multiprocessor machine
will build everything using spare resources
and check the result thereafter.
Since concurrency of make is not restricted
in subdirectories, this should be a nearly
optimal way to do it.

Also teach the user about a configure switch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47715 91177308-0d34-0410-b5e6-96231b3b80d8