permit wrapping expressions in the json file

the json parser passes line breaks verbatim, so we need to get rid of
them ourselves.

Change-Id: I3b71fe54dcaa8c3a2ff94e8ac4845f49dabe1663
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

mkspecs/features/qt_configure.prf

@@ -510,6 +510,7 @@ defineTest(qtRunSingleTest) {
 defineReplace(qtConfEvaluate) {
     isEmpty(1): return(true)
 
+    1 ~= s/$$escape_expand(\\n) */ /g
     expr = $${1}
     expr ~= s/&&/ && /g
     expr ~= s/\|\|/ || /g