Unicode string literals

[Marc Nieper-Wißkirchen]

On a system that supports at least C11, I can create an UTF8-encoded
literal string through:

(uint8_t const *) u8"..."

Could Gnulib abstract this into a macro so that substitutes for
systems that do not have u8 string literals can be provided.

On a C11 system, we would have

#define UTF8(s) ((uint8_t const *) u8 ## s)

and similar definitions for UTF16 and UTF32.

Similarly, something like

#define ASCII(s) (u8 ## s [0])

for pre-C2x systems would be nice so that ASCII("c") expands into the
ASCII code point of the character `c'.