Missing sources for target kconfigelektra

Did I miss a step from the new tutorial or is this really a bug? cmake reports an error when I try to compile our kconfig master branch

```sh
root@debian:/home/dominic/mount/kconfig/build# rm -rf *; cmake ..
-- The C compiler identification is GNU 9.2.1
-- The CXX compiler identification is GNU 9.2.1
-- 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
--

Installing in /usr/local. Run /home/dominic/mount/kconfig/build/prefix.sh to set the environment for KConfig.
-- Setting build type to 'Debug' as none was specified.
-- Looking for __GLIBC__
-- Looking for __GLIBC__ - found
-- Performing Test _OFFT_IS_64BIT
-- Performing Test _OFFT_IS_64BIT - Success
-- Performing Test HAVE_DATE_TIME
-- Performing Test HAVE_DATE_TIME - Success
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY - Success
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY - Success
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR - Success
-- Elektra 0.9.1 found
-- At least one python version must be available to use PythonModuleGeneration.
-- At least one python version must be available to use PythonModuleGeneration.
-- Elektra 0.9.1 found
-- The following OPTIONAL packages have been found:

* Elektra

-- The following REQUIRED packages have been found:

* ECM (required version >= 5.63.0), Extra CMake Modules.,
* Qt5Xml
* Qt5Gui
* Qt5DBus
* Qt5 (required version >= 5.11.0)
* Qt5Core

-- The following features have been disabled:

* QCH, API documentation in QCH format (for e.g. Qt Assistant, Qt Creator & KDevelop)

-- The following OPTIONAL packages have not been found:

* PythonModuleGeneration

-- Configuring done
CMake Error at /usr/local/share/ECM/modules/ECMAddTests.cmake:110 (add_executable):
No SOURCES given to target: kconfigelektra
Call Stack (most recent call first):
autotests/CMakeLists.txt:61 (ecm_add_test)

-- Build files have been written to: /home/dominic/mount/kconfig/build
```

According to git bisect this error has been introduced in b49010b118ba7c19830179edf6792bc174fcbf0e.

- cmake version 3.13.4
- Debian GNU/Linux bullseye/sid
- Elektra compiled and installed from tag 0.9.1

Reproduce the configuration failure with CMake, then inspect autotests/CMakeLists.txt at the ecm_add_test call on line 61 and commit b49010b118ba7c19830179edf6792bc174fcbf0e. Determine why kconfigelektra has no sources; done means the target is configured without the reported error.

Written by the indexing model from the issue text.