Make GSL easier consumed by another project.

[sezaru]

This PR is related to this issue: #363

Changes:

• Switched old include_directories function for the newer target_include_directories.
• Added GSL as a INTERFACE library
• Added GSL project to the tests CMakeLists.txt so It can include it automatically.

[msftclas]

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[msftclas]

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[sezaru]

Just for completeness, travis-ci is failing because this change requires CMake 2.8.12 instead of 2.8.7.

[neilmacintosh]

@sezaru it looks as though this PR would need work to resolve conflicts before we could apply it (as well as ensuring our Travis used right version of CMake). Are you interested in reviving it?

[neilmacintosh]

Given we now have #467 which appears to build on the ideas of this PR, I'll close this one out.