core guidelines support

[marcpawl]

Would be great to have GSL as an option.

[ddalcino]

This is a good idea, but it's trivial to add ms-gsl/3.1.0 or gsl-lite/0.37.0 to CONAN_EXTRA_REQUIRES.

[lefticus]

Agree with @ddalcino, that it's easy to add. Not sure if it's something that should be pushed by default or not?

That could be my own bias because I haven't used the gsl personally.