Allow build of at least the html docs when cross-compiling
Allow build of at least the html docs when cross-compiling

Qch requires qhelpgenerator which is not bootstrapped, but
qdoc works to some extent.

Change-Id: I08ce043281c45a29829a2ec25d8ad3e38e90d4e2
Reviewed-by: Jörg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@qt.io>

qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs