RFE: .NOT_PARALLEL extension to help with multiple targets

[Noel Yap]

I think this was mentioned on the list a month or so ago and I was against the 
idea at the time.  I've since changed my mind.

The RFE is that the following should be allowed:

  .NOT_PARALLEL: %.pm %_wrap.cc
  %.pm %_wrap.cc: %.swg
        $(SwigAction)

which would mean that %.pm and %_wrap.cc files should not be built in parallel 
to one another.  This would help in this situation since $(SwigAction) of %.swg 
will build %.pm and %_wrap.cc at the same time but the .NOT_PARALLEL would 
prevent the action
from being invoked twice.

There's probably lots missing here so any thoughts or comments would help.

Thanks,
Noel
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