Makefile: Workaround Borland Make bug with multiple outputs

Given a rule of the form

  out1 out2: dep1
  out1 out2: dep2

Borland Make complains that there are multiple rules for "out1"
even though this works when there is only one output.  Instead
generate

  out1 out2: dep1 dep2

for Borland Make, but only when there are multiple outputs.
Merge topic 'custom-command-multiple-outputs' into next

6c67b816 Makefile: Workaround Borland Make bug with multiple outputs