Bump required CMake version to 3.16
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KF6 task: https://phabricator.kde.org/T14467
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Remove CMAKE_AUTOMOC_RELAXED_MODE, deprecated since CMake 3.15
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set OpenGL_GL_PREFERENCE to LEGACY, with GLVND it fails to build on my system.
KF6 task: https://phabricator.kde.org/T14467
Remove CMAKE_AUTOMOC_RELAXED_MODE, deprecated since CMake 3.15
set OpenGL_GL_PREFERENCE to LEGACY, with GLVND it fails to build on my system.