qtwebengine build issues on windows

Currently testing the build of qtwebengine within vcpkg. While doing this I discovered some issues.

1. The build doesn't forward `<program>_EXECUTABLES` to the other CMake builds. In general the passing of variables seems to be incomplete. I am passing:

-DGPerf_EXECUTABLE=${GPERF}
 -DBISON_EXECUTABLE=${BISON}
 -DFLEX_EXECUTABLE=${FLEX}
 #-DGn_EXECUTABLE=${GN}
 -DPython2_EXECUTABLE=${PYTHON2}
 -DNodejs_EXECUTABLE=${NODEJS}

and had to patch the CMakeLists.txt to at least forward Python2_EXECUTABLE and Nodejs_EXECUTABLE. It seems all those executable need to be on PATH to be used.

1. GN is commented out since the build seems to only accept the GN build by Qt itself  as such it might be better to call it QtGN instead?
2. Passing Nodejs_EXECUTABLE seems to doing nothing except fixing a configure error.

I would expect the cmake build to setup the environment as needed for qtwebengine