The "*=" (conditional concatenate) operator in .pro files appears to be undocumented

Several operators exist to allow strings to be created or modified within a
.pro or .pri project file. These operators are:

= Assign
+= Concatenate
-= Remove
*= Conditional concatenate

The "*=" (conditional concatenate) operator isn't documented on this web
page describing .pro/.pri files and their syntax:

http://doc.trolltech.com/4.6/qmake-project-files.html