[REG 5.8 -> 5.9] ninja error when building in a path containing "+"

1. Clone Qt 5.9:

   git clone git://code.qt.io/qt/qt5.git qt-5.9.0-pre
   

2. Create a parallel directory which contains a "+" in its path.
   I have /home/qtrob/dev/g++-5.4.0/qt-5.9.0-pre
3. Try building Qt:

   nice /home/qtrob/dev/src/qt-5.9.0-pre/configure -developer-build -opensource -nomake examples -nomake tests -confirm-license && nice make -j8 && nice make -j8 docs && echo success
   

   Building WebEngine will fail:

   /home/qtrob/dev/g++-5.4.0/qt-5.9.0-pre/qtwebengine/src/3rdparty/ninja/ninja  -v -C /home/qtrob/dev/g++-5.4.0/qt-5.9.0-pre/qtwebengine/src/core/Debug QtWebEngineCore
   ninja: Entering directory `/home/qtrob/dev/g++-5.4.0/qt-5.9.0-pre/qtwebengine/src/core/Debug'
   ninja: error: obj/generate_cpp_mocs.ninja:1: expected newline, got lexing error
   rule _home_qtrob_dev_g++-5.4.0_qt-5.9.0-pre_qtwebengine_src_core_Debug_g...
                         ^ near here
   Makefile.gn_run:550: die Regel für Ziel „run_ninja“ scheiterte
   make[4]: *** [run_ninja] Fehler 1
   

Building WebEngine should be possible regardless of special characters in the path. Trying the same in directory /home/qtrob/dev/gxx-5.4.0/qt-5.9.0-pre does not lead to this error.