Improve Julia cmake build system
Summary: Update FindJulia.cmake according new version of this, add small change (change Sys to Libdl accroding this) 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
Differential Revision: https://phabricator.kde.org/D12121