Add support for toggling Fortran features

[awvwgk]

This has been hanging around stale long enough, here is a draft implementation for disabling implicit typing and external interfaces by default with opt-in toggles on per package basis. Also allows to switch globally between fixed and free format for source files.

• implicit-typing: toggle default implicit typing rules
  • option to disable in GFortran
  • option to enable in LFortran
• implicit-external: toggle implicit external interfaces
  • option to disable in GFortran
  • option to enable in LFortran
• source-form: select source form ("free", "fixed", or "default")
  • option to set free/fixed form in GFortran
  • option to set fixed form in LFortran

Usage

[fortran]
implicit-typing = true  # default: false
implicit-external = true  # default: false
source-form = "fixed"  # default: "free"

Documentation at fortran-lang/fpm-docs#98
Closes #577
Closes #363

[gnikit]

Very nice you beat me to it (by many, many hours). I left some comments/patch suggestions for adding support for the other compilers (at least for the fixed/free flag). Have a look and let me know if they are correct, if not you can dismiss them.

[awvwgk]

Giannis @gnikit, feel free to commit the changes directly to my branch.

[gnikit]

Giannis @gnikit, feel free to commit the changes directly to my branch.

Will do this today

[everythingfunctional]

Haven't had a chance to look at the actual code changes, but I think your description is a reasonable approach. My only comment is about the "default" for source-form. I'm not sure that's exactly the right spelling for that option. Perhaps "mixed" or "detected" or "assumed" would be better?

[milancurcic]

I think this is a great change.

The TOML key should be source-form, not source-format. Likewise, use "fixed-form" and "free-form" instead of "fixed-format" and "free-format" in example names.

"default" for the source-form makes sense to me--it reads like "use the compiler default, whatever that is".

Added suggested compiler options

[awvwgk]

Thanks everyone for reviewing and providing feedback. While not all compilers currently support these features, we can add those in the future as we discover the correct options. I'll go ahead and merge now.