Case-insensitive ext4

Well, CMake has "is case sensitive" logic baked in at compile time. Apple and Windows are "always case insensitive" and everything else is always case sensitive. I don't know what kinds of changes would be required in build tools to do this case insensitive comparisons. For example, this just doesn't work with ninja on Windows (I assume make has similar issues with the analogous ruleset):

rule copy
command = cp $in $out
build foo: copy in
build bar: copy FOO

saying that no rule makes FOO even though technically it will exist if you build foo. Basically, build tools that exist today need cases to match everywhere. And yes, ninja could figure this out right now, but if `dir/foo` and `dir/FOO` is used and `dir` is made by some rule during the build, its case sensitive flag can't be known at the start.

Case insensitivity in filesystems is broken. Conditional case sensitivity at a per-filesystem level means even ninja needs to add ioctl queries to figure that out, but `--one-file-system` is something that is at least enforceable. Per-directory flags which require magical "what will the flag on this directory be in the future" is even more broken.

I'd be surprised if "doesn't work in case insensitive ext4 directories" (nevermind an environment with a mix of case sensitive and insensitive directories) issues don't get closed as WONTFIX in many tools.