Then you're better off using custom types - that way people will immediate know your type is non-default - as opposed to hiding your customization away in a makefile, pranking people who expect built-ins to behave a certain way.

The people who understand that it can be either, depending on a compiler switch, are exactly the people who use an explicit sign (typically via a typedef) to ensure their code always works.

The people who say that char is de facto signed and everyone should just deal with it, are the people who end up writing broken code.

"might" be able to change it. It's fine in GCC based compilers, but some awkward/old compilers don't give you the luxury

Um, it was dependent on how the CPU handled it back in those days. That problem went away, though.