[REG 5.6.2 -> 5.6] Compile error: QtQml/qqml.h not found

1. Check out Qt 5.6 branch.
2. Configure in a shadow build directory:

   /home/qtrob/dev/src/qt-5.6.3-pre/configure -developer-build -opensource -nomake examples -nomake tests -confirm-license -no-gtkstyle
   

3. make
   This results in a compile error:

   make[3]: Verzeichnis „/home/qtrob/dev/g++-6.2.0/qt-5.6.3-pre/qtwebchannel/src/webchannel“ wird betreten
   g++ -c -pipe -g -std=c++1z -fvisibility=hidden -fvisibility-inlines-hidden -Wall -W -Werror -Wno-error=cpp -Wno-error=deprecated-declarations -Wno-error=strict-overflow -fno-exceptions -D_REENTRANT -fPIC -DQT_NO_TSLIB -DQT_NO_JSVALUE -DQT_BUILD_WEBCHANNEL_LIB -DQT_BUILDING_QT -DQT_NO_CAST_TO_ASCII -DQT_ASCII_CAST_WARNINGS -DQT_MOC_COMPAT -DQT_USE_QSTRINGBUILDER -DQT_DEPRECATED_WARNINGS -DQT_DISABLE_DEPRECATED_BEFORE=0x050000 -DQT_NO_EXCEPTIONS -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -DQT_CORE_LIB -Wall -Wextra -Werror -Woverloaded-virtual -Wshadow -Wundef -Wnon-virtual-dtor -Wpointer-arith -Wformat-security -Wno-long-long -Wno-variadic-macros -pedantic-errors -Wchar-subscripts -Wcast-align -Wzero-as-null-pointer-constant -I/home/qtrob/dev/src/qt-5.6.3-pre/qtwebchannel/src/webchannel -I. -I/home/qtrob/dev/g++-6.2.0/qt-5.6.3-pre/qtbase/include -I/home/qtrob/dev/g++-6.2.0/qt-5.6.3-pre/qtbase/include/QtWebChannel -I/home/qtrob/dev/g++-6.2.0/qt-5.6.3-pre/qtbase/include/QtWebChannel/5.6.2 -I/home/qtrob/dev/g++-6.2.0/qt-5.6.3-pre/qtbase/include/QtWebChannel/5.6.2/QtWebChannel -I/home/qtrob/dev/g++-6.2.0/qt-5.6.3-pre/qtbase/include/QtCore/5.6.2 -I/home/qtrob/dev/g++-6.2.0/qt-5.6.3-pre/qtbase/include/QtCore/5.6.2/QtCore -I/home/qtrob/dev/g++-6.2.0/qt-5.6.3-pre/qtbase/include/QtCore -I.moc -I/home/qtrob/dev/src/qt-5.6.3-pre/qtbase/mkspecs/linux-g++ -DQT_NO_CAST_TO_ASCII=1 -DQT_NO_CAST_FROM_ASCII=1 -DQT_STRICT_ITERATORS -DQT_NO_URL_CAST_FROM_STRING=1 -DQT_NO_CAST_FROM_BYTEARRAY=1 -DQT_NO_KEYWORDS=1 -DQT_USE_FAST_CONCATENATION -DQT_USE_FAST_OPERATOR_PLUS -Dsignals=int -Dslots=int -Demit=public: -Dforeach=public: -Dforever=public: -xc++ /home/qtrob/dev/src/qt-5.6.3-pre/qtwebchannel/src/webchannel/qqmlwebchannel.h -o .obj/header_qqmlwebchannel.o
   /home/qtrob/dev/src/qt-5.6.3-pre/qtwebchannel/src/webchannel/qqmlwebchannel.h:40:24: fatal error: QtQml/qqml.h: Datei oder Verzeichnis nicht gefunden
    #include <QtQml/qqml.h>
                           ^
   compilation terminated.
   Makefile:1256: die Regel für Ziel „.obj/header_qqmlwebchannel.o“ scheiterte
   

It should be possible to compile Qt.