Ensure that qcollectiongenerator is called from the correct directory.

Even though QT_BINARY_DIR is specified, cmake will try PATH first.

Gentoo-bug: 544108

--- a/src/doc/CMakeLists.txt
+++ b/src/doc/CMakeLists.txt
@@ -9,6 +9,7 @@
 FIND_PROGRAM(QT_HELP_COLLECTION_GENERATOR
   qcollectiongenerator
   PATHS "${QT_BINARY_DIR}"
+  NO_DEFAULT_PATH
   DOC "qcollectiongenerator used to compile Qt help collection project files")
 
 ADD_CUSTOM_COMMAND(