add GHA for compilation and testing

[ocefpaf]

@efiring I may need some help on how you test this locally. We do not have a make check, right?

[efiring]

There is no "make check", just the gsw_check executable. It currently returns zero regardless of whether it fails or succeeds. Making it return 1 (for example) on failure requires changing

        if (gsw_error_flag)
            printf("\nYour installation of the Gibbs SeaWater (GSW) "
                "Oceanographic Toolbox has errors !\n");
        else

to

        if (gsw_error_flag)
        {
            printf("\nYour installation of the Gibbs SeaWater (GSW) "
                "Oceanographic Toolbox has errors !\n");
            return (1);
        }
        else

in gsw_check_functions.c.

[ocefpaf]

I can make the changes later this week. Working on setting the compiler config up first.

[DocOtak]

I took a crack at having a non zero exit status in #43

If you want just cherrypick that code and stick it in this PR.

[ocefpaf]

It is late for me and I'm on my phone only now. It is OK to merge your PR and I'll rebase this PR tomorrow.

Thnaks!

[efiring]

For Linux/OSX we need to run "make -f Makefile", not just plain "make", so that Makefile will be used instead of GNUmakefile. Or remove (or move) GNUmakefile, which is obsolete; it has features specific to its origin at SIO. It shows how to do some things that someone might want, so instead of deleting it, perhaps it should be moved to "GNUmakefile.example".

[ocefpaf]

@DocOtak and @efiring this is ready for review. I tested it by breaking the code and the CIs failed as expected. Thanks @DocOtak for #43!

PS: the Windows one is also working. I have a draft in #44