Support building from source with user-specified GraphBLAS

[alugowski]

Look for GraphBLAS first in GraphBLAS_ROOT env var, if empty then fallback to current paths.

using GraphBLAS_ROOT because that's what CMake expects when building against GraphBLAS in C++.

fixes #79

[eriknw]

Thanks @alugowski! I'm going to give this a try locally and will let you know how it goes.

Btw, we do squash merges by default, so there is little need to rebase and force push to change history (but you can still do so if you want).

[eriknw]

Great, this works for me!

The graphblas shared objects also need to be findable at runtime. If SuiteSparse:GraphBLAS is installed in a non-standard location, this probably requires LD_LIBRARY_PATH to be set appropriately. I'm not sure if this is worth mentioning in the readme (if a user is compiling from source, they probably know this).

[alugowski]

Good point, LD_LIBRARY_PATH is easy to forget about.

If this is a common use then supporting static linking may be useful. Fewer frictions for the end user is always better.

[eriknw]

Thanks for the quick update, this looks great to me!

I'd rather leave worrying about static linking for another day ;)

Merging and then doing an alpha release to test things out 🤞 🚀

[alugowski]

Sounds good!