Tidying on external_deps

[illwieckz]

Tidying on external_deps.

Extracted from:

• Rework external_deps for Saigo and CMake-based NaCl rebuild #1986

The unify style commit is just bikeshedding, it's just to be consistent within the file (not claiming that new files should do it that way), so later code I already wrote will also be consistent
The rewriting comment commit is also just bikeshedding, but future architecture implementations (already written) will match that wording.

The smart_copy commit is useful as it uses copy-on-write enabled cp when available (much faster copy because data is not duplicated on disk). The copy-on-write enabled cp already knows how to do a standard copy when the feature is not provided by the filesystem, the test to fallback on the standard cp tool on systems like macOS.

Some commits rewrite the way things are copied, in better way (for example to avoid globbing when unneeded).
In some cases, it actually fixes a bug where a folder is copied in itself the second time the script is run. This only applied to a software we don't package yet (WASI), but letter have this ready for the day we continue this effort.

The EXE_EXT thing will reduce diff noise when future code will land.

The xz compression stuff should be obvious, it's just an extra option to make the compression more extreme.

We don't have to wait for more meaningful changes to merge that, and it makes my branch smaller.

[illwieckz]

I forgot to say may the smart_copy commit also unifies the used copy options (and avoid some GNUism).