Install SuiteSparse headers in subdirectory by default

[mmuetzel]

This change would install (most of) the headers of SuiteSparse libraries to a subfolder suitesparse of the include folder.
That would prevent accidental inclusion of older SuiteSparse headers when the system include directory is added to the preprocessor search path. See #558.

[DrTimothyAldenDavis]

I will give it a try. I can see that this could help avoid conflicts with /usr/local/include (my omp.h is there). So this is probably a good thing to add. If a package manager wants to move them elsewhere, they can do that themselves.

But if omp.h is in /opt/homebrew/include, alongside a stale amd.h from "brew install suite-sparse", then that will still cause a problem.

[mmuetzel]

I will give it a try. I can see that this could help avoid conflicts with /usr/local/include (my omp.h is there). So this is probably a good thing to add. If a package manager wants to move them elsewhere, they can do that themselves.

But if omp.h is in /opt/homebrew/include, alongside a stale amd.h from "brew install suite-sparse", then that will still cause a problem.

Yes, this won't fix it when the installed SuiteSparse headers are already mixed with the headers from other libraries. But it might make life easier at some point in the future.