Building QtPdf is prevented for no obvious reason

Building QtPdf is being prevented for an unknown reason.  When configuring there is a message stating that the dependent FEATURE_qtpdf_build is marked as dirty. See attached the configure output given when configuring through QtCreator. The Cmake cache has been deleted prior to configuring as well.

Here is a small extract:

[QtBase]     'qtpdf_build' was changed from OFF to ON
[cmake] [QtBase]     'qtpdf_widgets_build' was changed from OFF to ON
[cmake] – [QtWebEngine] Configure checks for qtpdf failed.
[cmake]   Resetting 'FEATURE_qtpdf_build' from 'ON' to 'OFF' because it doesn't meet
[cmake]   'QT_CONFIGURE_CHECK_qtpdf_build'
[cmake]   Auto-resetting 'FEATURE_qtpdf_widgets_build' from 'ON' to 'OFF', because
[cmake]   the dependent feature 'qtpdf_build' was marked dirty.
[cmake]   Build QtPdf Modules:
[cmake]     Build QtPdfWidgets ................... no
[cmake]     Build QtPdfQuick ..................... no
[cmake] WARNING: QtPdf won't be built.