I found the answer on Stackoverflow. Looks like the answer is to do this in the .pro file:

Qt Code: 
  1. CONFIG -= debug_and_release
  2. CONFIG( debug, debug|release )  {
  3.   CONFIG -= release
  4. }
  5. else {
  6.   CONFIG -= debug
  7.   CONFIG += release
  8. }
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This seems to work , but Yuk! Surely there must be a more elegant way?