WIN64: 'Environment variable 'QT_INSTALL_HEADERS' undefined' error for QtWinExtras documentation builds

        cd qtwinextras\ && ( if not exist Makefile C:\libQT-5.11-dev\build\qtbase\bin\qmake.exe -o Makefile C:\libQT-5.11-dev\build\qtwinextras\qtwinextras.pro ) && C:\Windows\jom.exe -f Makefile prepare_docs
C:/libQT-5.11-dev/build/qtwinextras/src/winextras/doc/qtwinextras.qdocconf:16: (qdoc) error: Environment variable 'QT_INSTALL_HEADERS' undefined
        cd src\ && ( if not exist Makefile C:\libQT-5.11-dev\build\qtbase\bin\qmake.exe -o Makefile C:\libQT-5.11-dev\build\qtwinextras\src\src.pro ) && C:\Windows\jom.exe -f Makefile prepare_docs
        cd winextras\ && ( if not exist Makefile C:\libQT-5.11-dev\build\qtbase\bin\qmake.exe -o Makefile C:\libQT-5.11-dev\build\qtwinextras\src\winextras\winextras.pro ) && C:\Windows\jom.exe -f Makefile prepare_docs
        C:\Windows\jom.exe -f Makefile.Debug prepare_docs
        call C:\libQT-5.11-dev\build\qtwinextras\src\winextras\qtattributionsscanner_wrapper.bat C:/libQT-5.11-dev/build/qtwinextras --filter QDocModule=qtwinextras -o C:/libQT-5.11-dev/build/qtwinextras/src/winextras/codeattributions.qdoc
        call C:\libQT-5.11-dev\build\qtwinextras\src\winextras\qdoc_wrapper.bat -outputdir C:/libQT-5.11-dev/build/qtwinextras/doc/qtwinextras -installdir C:/libQT-5.11-dev/MSVC64DH/doc C:/libQT-5.11-dev/build/qtwinextras/src/winextras/doc/qtwinextras.qdocconf -prepare -indexdir C:/libQT-5.11-dev/build/qtbase/doc -no-link-errors -I. -I..\..\include -I..\..\include\QtWinExtras -I..\..\include\QtWinExtras\5.12.0 -I..\..\include\QtWinExtras\5.12.0\QtWinExtras -Itmp -IC:\libQT-5.11-dev\build\qtbase\include\QtGui\5.12.0 -IC:\libQT-5.11-dev\build\qtbase\include\QtGui\5.12.0\QtGui -IC:\libQT-5.11-dev\build\qtbase\include -IC:\libQT-5.11-dev\build\qtbase\include\QtGui -IC:\libQT-5.11-dev\build\qtbase\include\QtANGLE -IC:\libQT-5.11-dev\build\qtbase\include\QtCore\5.12.0 -IC:\libQT-5.11-dev\build\qtbase\include\QtCore\5.12.0\QtCore -IC:\libQT-5.11-dev\build\qtbase\include\QtCore -I.moc\debug -I"C:\Program Files (x86)\VulkanSDK\include" -IC:\libQT-5.11-dev\build\qtbase\mkspecs\win32-msvc
Environment variable 'QT_INSTALL_HEADERS' undefined

Aborting
jom: C:\libQT-5.11-dev\build\qtwinextras\src\winextras\Makefile.Debug [prepare_docs] Error 1
jom: C:\libQT-5.11-dev\build\qtwinextras\src\winextras\Makefile [debug-prepare_docs] Error 2
jom: C:\libQT-5.11-dev\build\qtwinextras\src\Makefile [sub-winextras-prepare_docs_ordered] Error 2
jom: C:\libQT-5.11-dev\build\qtwinextras\Makefile [sub-src-prepare_docs] Error 2