Add OPENSURGSIM_DATA_DIR to the exported variables

This way other application can get access to the default shaders
Fix path on linux
Install Data into install target
Add specific linux data directory
Fix cmake syntax error
Different Path for Data install directory
Merge branch 'feature/add-data-directory' into 'master'

Add a cmake variable to point to the OSS data directory

+1: Paul Novotny, haizhouW, Julien Lenoir

Merged-on: https://www.assembla.com/code/OpenSurgSim/git/merge_requests/1062743