Implement <system> include for the standalone wrapper

[mmmhj2]

This pull request adds #include <system> directive support for the standalone wrapper as specified in the ARB_shading_language_include extension.

The current spec seems to give the implementation a lot of liberties on how to resolve the inclusion paths, similar to the C/C++ standard. This PR implements this directive similar to the GCC preprocessor:

• "local" inclusions will be searched first in the current source directory recursively, and then external directories specified by -I flag, in left-to-right order;
• <system> inclusions will be searched in external directories in left-to-right order only.

[CLAassistant]

All committers have signed the CLA.

[dneto0]

This should be testable. Please add tests.

[mmmhj2]

I have added a test, please check if it is sufficient.

I'm not entirely sure about the local inclusion search algorithm currently implemented though. It will search in the directories of the includers if not found in current source directory (see Test/inc1/foo.h, which includes Test/parent.h). I doubt whether this is how C/C++ preprocessors work, but I have reused this algorithm for system inclusions.

[dnovillo]

Thanks for the improvement! Some test suggestions:

• A GLSL test
• A negative test for unresolved system includes.

[mmmhj2]

Thanks for the improvement! Some test suggestions:

* A GLSL test

* A negative test for unresolved system includes.

Two more tests are added! Please tell me if more or more robust tests are needed.

[dnovillo]

LGTM. Thanks for the new tests.