qdoc tool is not generated during Qt installation via source files

$ su -
# mkdir /opt/qt-5.11.1 ; chown user:user /opt/qt-5.11.1
# exit
$ cd /tmp
$ tar xJvf qt-everywhere-src-5.11.1.tar.xz
$ cd qt-everywhere-src-5.11.1
$ ./configure -prefix /opt/qt-5.11.1 -opensource -confirm-license -release -shared -sql-psql -sqlite -qpa xcb -opengl es2 -qtnamespace Qt5 -no-directfb -dbus-linked -nomake tests -recheck-all -openssl-linked -libproxy -sctp
$ make -j4
$ make install
$ export PATH=/opt/qt-5.11.1/bin:$PATH
$ export LD_LIBRARY_PATH=/opt/qt-5.11.1/lib:$LD_LIBRARY_PATH
$ make docs

[...see log file attached...]

/tmp/qt-everywhere-src-5.11.1/qtbase/src/corelib/qdoc_wrapper.sh: 12: exec: /opt/qt-5.11.1/bin/qdoc: not found

Makefile:1699: recipe for target 'prepare_docs' failed make[5]: *** [prepare_docs] Error 127

make[5]: Leaving directory '/tmp/qt-everywhere-src-5.11.1/qtbase/src/corelib'

Makefile:1366: recipe for target 'sub-corelib-prepare_docs' failed

make[4]: *** [sub-corelib-prepare_docs] Error 2

make[4]: Leaving directory '/tmp/qt-everywhere-src-5.11.1/qtbase/src'

Makefile:431: recipe for target 'sub-src-prepare_docs' failed make[3]: *** [sub-src-prepare_docs] Error 2

make[3]: Leaving directory '/tmp/qt-everywhere-src-5.11.1/qtbase'

Makefile:2077: recipe for target 'module-qtbase-prepare_docs' failed

make[2]: *** [module-qtbase-prepare_docs] Error 2

make[2]: Leaving directory '/tmp/qt-everywhere-src-5.11.1'

Makefile:1217: recipe for target 'html_docs' failed

make[1]: *** [html_docs] Error 2

make[1]: Leaving directory '/tmp/qt-everywhere-src-5.11.1'

Makefile:1220: recipe for target 'docs' failed

make: *** [docs] Error 2