Improve Julia cmake build system

Summary: Update `FindJulia.cmake` according new version of [this](https://gist.github.com/JayKickliter/06d0e7c4f84ef7ccc7a9), add small change (change `Sys` to `Libdl` accroding [this](https://github.com/JuliaLang/julia/pull/10328)) and add checking of existing dev files of julia, because you can have julia, but haven't include files. Also fix some problems in `juliabackends` cmake files, related with case sensivity of cmake variables.

Reviewers: #cantor, asemke

Reviewed By: #cantor, asemke

Subscribers: #cantor, #kde_edu

Tags: #kde_edu, #cantor

Differential Revision: https://phabricator.kde.org/D12121