nlohmann: fix use of unsupported CMake variable

[olafmandel]

For CMake versions >=3.15, <3.17 the CMAKE_CURRENT_FUNCTION_LIST_DIR variable is not yet available, so searching for the local nlohmann header failes. Fix this by adding a workaround variable KKDockWidgets_PROJECT_ROOT that allows finding the correct directory anyway.

[iamsergio]

Hi, thank you for the contribution. Have you signed the CLA ? See https://kdab.github.io/KDDockWidgets/involved.html thanks

[CLAassistant]

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[olafmandel]

Have you signed the CLA ? See https://kdab.github.io/KDDockWidgets/involved.html thanks

Already signed and sent in, see my eMail on 2023-01-16 to info@kdab.com

[iamsergio]

found it! thank you