Support full paths in FC

[ScottWales]

Allow FC to be a full compiler path

Use LD for the linker in zero-config builds

Fixes #275

[MatthewHambley]

This is a good change which tackles a known usage pattern. There is still the issue of compiler executables being versioned, e.g. myfortran-x.y.z but that can be left 'till later.

[james-bruten-mo]

This looks sensible to me and I can see you've added the Optional label requested by Matthew so I think everything is good

[james-bruten-mo]

Hi Scott, the build tests on the merge are failing with Argument 1 to "_generic_build_config" has incompatible type "Union[Path, Any, None]"; expected "Path". I assume you now only want to call _generic_build_config from cli_fab when folder is not None. Cheers

[ScottWales]

Fixed up the typing (the Optional[] on folder is needed to make the command line entry_point work)

Changes requested have been made so dismissing this review

[james-bruten-mo]

Changes now pass unit tests, thanks