No need to call declare -p twice. The case statement is easier to read
than the multi-if.

structuredAttrs was used here to make an assumption about the type of
the named variables passed as arguments.  This can be done better by
looking at the actual types of those variables.

This gives a bit more backwards compatibility as well: Once you turn to
structuredAttrs, you should still be able to pass a bare string instead
of a list and have it behave as a whitespace-separated string like
before.

Passing "flagsArray" as the first argument allows using this function
in a few more places.

Instead of checking for __structuredAttrs everywhere, it's easier to
just set the default value via parameter expansion and then hand the
array construction off to "concatTo".

Once more setup-hooks will be made structuredAttrs-aware, this pattern
will reduce the use of this implementation detail even more.

This can be used to separate lists for example with commas, when
creating argument strings. This works with both structuredAttrs disabled
and enabled.

Tested emilua with and without __structuredAttrs.

This was used in #NixOS#35666 to add the --cross-file argument, but NixOS#86080
moved that somewhere else - leaving crossMesonFlags unused.

Tested clasp-common-lisp with and without __structuredAttrs.

Tested litehtml with and without __structuredAttrs.

Resolves NixOS#289037
Supersedes NixOS#299622 (at least parts)

Tested db with and without __structuredAttrs.

The test fails without 64eaa63
("stdenv: concatStringsSep: quote ${sep}")

Co-authored-by: Ivan Trubach <mr.trubach@icloud.com>
Co-authored-by: Wolfgang Walther <walther@technowledgy.de>