qtdoc build disabled in 5.11/dev

After https://codereview.qt-project.org/#/c/151666/ landed, the build of qtdoc became disabled on all platforms (despite this being pointed out in the review and instead ignored).

This is visible in the individual qttools integrations in coin where the qtdoc build is skipped and it is also visible in the qt5.git integration failing:

https://testresults.qt.io/coin/integration/qt/qt5/tasks/1502857328
http://coin.intra.qt.io/coin/integration/qt/qt5/tasks/1502857328

In this example build of a qt5 integration there are many items red, but for this issue the qt5 Ubuntu 16.04 build is relevant:

http://coin.intra.qt.io/coin/api/results/qt/qt5/64a9b0e21eb55b129d5eaabc106571444e83febf/LinuxUbuntu_16_04x86_64LinuxUbuntu_16_04x86_64GCCqtci-linux-Ubuntu-16.04-x86_64-1-bbe656DeveloperBuild_OutOfSourceBuild_QtLibInfix_QtNamespace/2a48a3b7dcb9652b1532a3ad14de7580b5b9b1ee/build_1502857439/buildlog.txt.gz

/home/qt/work/build/qtbase/qmake/qdoc_wrapper.sh: 12: exec: /home/qt/work/build/qtbase/bin/qdoc: not found

A binary of qtdoc is needed in order to build the documentation of Qt for the binary packages we release, which happens as part of the integration of qt5.git.

Consequently there are two issues that need to be addressed:

(1) We need to build qdoc binaries in all the packaging configurations in order to continue to ship qdoc binaries to our customers.
(2) We need to build qdoc binaries in the qt5.git configuration that's used to build documentation. In dev that's Ubuntu 16.04 right now.

For both to succeed it is necessary to have clang installed in the environment the qttools module is built for (1) and (2).

A successful build of the documentation during a qt5 integration is required to release, therefore this has P0 blocking priority. We cannot release Qt without generated documentation.