BUILD: compiler: define a __fallthrough statement for switch/case

BUILD: compiler: define a __fallthrough statement for switch/case

When the code is preprocessed first and compiled later, such as when
built under distcc, a lot of fallthrough warnings are emitted because
the preprocessor has already stripped the comments.

As an alternative, a "fallthrough" attribute was added with the same
compilers as those which started to emit those warnings. However it's
not portable to older compilers. Let's just define a __fallthrough
statement that corresponds to this attribute on supported compilers
and only switches to the classical empty do {} while (0) on other ones.

This way the code will support being cleaned up using __fallthrough.