If LSCOLORS is undefined or partially defined (missing some of the eleven expected
value pairs), the patch will automatically fill in those missing values with default
colors. However, if the LS_COLORS environment variable is already defined, the
application prefer the LS_COLORS approach, maintaining current working flow of the
program.

Support for FreeBSD-style ls Coloring

There are some false positives, so it's not added to distcheck yet.

Link: https://developer.apple.com/documentation/technotes/tn3150-getting-ready-for-data-less-files

BSD/macOS ls(1) don't reject overlong or odd-length $LSCOLORS values
(although they can warn, which we don't).

They also don't use the "intense" background colors for capital letters;
instead, that enables underline on FreeBSD, or (foreground) bold on
macOS.  We copy FreeBSD here.

countof() will likely be in the C2Y standard.

Link: https://www.open-std.org/JTC1/SC22/WG14/www/docs/n3469.htm
Link: https://thephd.dev/c2y-hitting-the-ground-running

For broader compiler support, we try both -std=c23 and -std=c2x.  If
neither is supported, we fall back to -std=c17, but we will start
requiring some C23 features as extensions in this mode.

We can avoid the MACRO_((list), __VA_ARGS__, ) dance since the comma is
no longer required.  typeof() also comes in handy.