This explicitly spells out what informally has been the coding
convention for some time on preprocessor directives, making it
more transparent and easier to find.
This is particularly useful as the code base has at least three
different styles. Deducing what actually is the current policy
would require a details look at how the style has changed over
time, rather than being obvious from a quick `grep` over the
code base.
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.
