Only use macOS sheet modals when compiler supports blocks

[williamsjoblom]

Blocks are a non-standard extension and are not supported by GCC. Sheet
modals require blocks, so let's fall back on a regular modal when blocks
are not supported.

See GCC discussion about block support: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78352

[Albrecht-S]

Thanks for the report and PR. This looks like something for @ManoloFLTK

[ManoloFLTK]

Good point. I've committed that, in a slightly different form, to 77b3557.
Thanks.

[Albrecht-S]

@ManoloFLTK What if __BLOCKS__ is not defined? A picky compiler (or one with such warnings enabled) would issue a warning. If you really need to check that __BLOCKS__ is true (if defined) you would need both conditions which would be evaluated left to right as usual. Or am I missing anything?

[ManoloFLTK]

OK It's better to use defined(BLOCKS). Committed at b6b1fcc.

[williamsjoblom]

I just wanted to give you (@ManoloFLTK) a heads up that setting the author field to the actual author is considered good practice when you, as a maintainer, did no modifications to the code. Since my change is so small I don't really care, but these things often tend to spark a fair bit of controversy for bigger contributions.