From Baptiste Jonglez https://tuleap.ring.cx/plugins/tracker/?aid=1887

Arch Linux recently updated to Qt 5.11, and it broke builds for LRC.

Here is the output of CMake:

-- The C compiler identification is GNU 8.1.1
-- The CXX compiler identification is GNU 8.1.1
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found GCC version >= 4.8: 8.1.1

-- Ring daemon header is in /usr/include/dring
-- Ring library path is /usr/lib/libring.so
-- VIDEO enabled
-- Using Ring DBus-XML interfaces in /usr/share/dbus-1/interfaces
CMake Error at CMakeLists.txt:710 (QT5_USE_MODULES):
Unknown CMake command "QT5_USE_MODULES".

According to the doc https://doc.qt.io/qt-5/cmake-manual.html , QT5_USE_MODULES is deprecated, so it probably got removed in Qt 5.11. The doc also says that TARGET_LINK_LIBRARIES should be used instead:

qt5_use_modules(target [LINK_PUBLIC|LINK_PRIVATE] module ... ) Indicates that the target uses the named Qt 5 modules. The target will be linked to the specified modules, use the include directories installed by those modules, use the COMPILE_DEFINITIONS set by those modules, and use the COMPILE_FLAGS set by the modules. The LINK_PRIVATE or LINK_PUBLIC specifiers can optionally be specified. If LINK_PRIVATE is specified then the modules are not made part of the link interface of the target. See the documentation for target_link_libraries for more information.

Note that this macro is only available if using CMake 2.8.9 or later. This macro is obsolete. Use target_link_libraries with IMPORTED targets instead.