qtwebengine: add option to build gn only

IIUC, when cross-building QtPdf for android, it requires that the host build installs gn.

So I have to configure & build qtwebengine completely just to get that binary on the host.

I know there is   

  -DFEATURE_qtpdf_build=OFF
  -DFEATURE_qtwebengine_build=OFF

But if both are disabled, it won't build gn (see src/CMakeLists.txt)

if(NOT QT_FEATURE_qtwebengine_build AND NOT QT_FEATURE_qtpdf_build)
    return()
endif()

Would it be possible to have an option to build only "gn" when building the host, or build let gn be built even if we have both   -DFEATURE_qtpdf_build=OFF -DFEATURE_qtwebengine_build=OFF

Ideally, the required dependencies would be updated too to have only those really required, and not require all the nodejs & so stuff (that is currently required for qtwebengine) if actually not mandatory to build gn.