This makes an implicit agreement that code should include the headers
is uses, no more and no less, explicit.
It also recommends using tooling (such as clangd) and adding IWUY
pragma comments to aid those tools in common cases they otherwise would
provide false positives.
Co-authored-by: chrysn <chrysn@fsfe.org>
The standard is 9 years old now, it is well supported in all mayor compilers.
In fact, features of the 'new' standard are already used in RIOT (std_atomic).
Let's make it the default and adapt the Makefiles accordingly.
