Fix glad add_subdirectory to fix out of tree builds
Fix glad add_subdirectory to fix out of tree builds

This was a problem on the arrayfire-benchmark repo where the repository
is built as a subproject

umar456/arrayfire,umar456/arrayfire,arrayfire/arrayfire,arrayfire/arrayfire,arrayfire/arrayfire,9prady9/arrayfire,arrayfire/arrayfire,umar456/arrayfire,umar456/arrayfire,9prady9/arrayfire,9prady9/arrayfire,9prady9/arrayfire