On Windows, currently it is not possible to customize CMAKE_INSTALL_DATAROOTDIR, see https://sourceforge.net/p/lensfun/code/ci/b9540f30f880e723d51cd1438ef1b7506251309f/tree/CMakeLists.txt#l58. To be consistent with other operating systems, this should be guarded like IF(WIN32 AND NOT DEFINED CMAKE_INSTALL_DATAROOTDIR)
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Also fixed in master. Thanks!