qtwebengine-5.12.0 does not build except as part of full qt

On Beyond Linux From Scratch we have been building Qt5 without webengine, and then building webengine in separate instructions for the few of us who need it. This saves a lot of time, and a little space, for people who do not use falkon. It also allows our automated 'jhalfs' process to pick the correct dependencies so that falkon can be built (the alternative would be to only mention webengine in text notes, which will prevent the automated build).

Up to 5.11.3 this works. In 5.12.0, the build fails although building the whole package works, i.e. building webengine within qt5 instead of trying to build the separate qtwebengine tarball afterwards).

In target 15580/16103 which appears to be compiling client_cert_select_controller.cpp

In file included from /scratch/ken/qtwebengine-everywhere-src-5.12.0/src/core/client_cert_select_controller.cpp:40:
/scratch/ken/qtwebengine-everywhere-src-5.12.0/src/core/client_cert_select_controller.h:75:37: error: variable 'QWEBENGINECORE_EXPORT ClientCertSelectController' has initializer but incomplete type
class QWEBENGINECORE_PRIVATE_EXPORT ClientCertSelectController {
^~~~~~~~~~~~~~~~~~~~~~~~~~

Gentoo are using a sed,

find . -type f -name "*.pr[fio]" |
xargs sed -i -e 's|INCLUDEPATH += |&$$QTWEBENGINE_ROOT/include |'

(in practice, prf fiels are not altered by that)

With that, the build almost completes. In the final target when it tries to link ../../libexec/QtWebEngineProcess it reports

.obj/main.o:main.cpp:function localtime: error: undefined reference to 'sandbox::localtime_override(long const*)'

That appears to be present in /build/src/core/release/obj/sandbox/linux/libsandbox_services.a and that archive is specified among the files to be linked into libQtWebEngineCore.so.5.12