😃 (and other unicode characters) in identifiers not allowed by g++

I am 😞 to find that I cannot use 😃 as a valid identifier with g++ 4.7, even with the -fextended-identifiers option enabled:

int main(int argc, const char* argv[]){  const char* 😃 = "I'm very happy";  return 0;}

main.cpp:3:3: error: stray ‘\360’ in program
main.cpp:3:3: error: stray ‘\237’ in program
main.cpp:3:3: error: stray ‘\230’ in program
main.cpp:3:3: error: stray ‘\203’ in program

After some googling, I discovered that UTF-8 characters are not yet supported in identifiers but a universal-character-name should work. So I convert my source to:

int main(int argc, const char* argv[]){  const char* \U0001F603 = "I'm very happy";  return 0;}

main.cpp:3:15: error: universal character \U0001F603 is not valid in an identifier

So apparently 😃 isn't a valid identifier character. However, the standard specifically allows characters from the range 10000-1FFFD in Annex E.1 and doesn't disallow it as an initial character in E.2. My next effort was to see if any other allowed unicode characters worked - but none that I tried did. Not even the ever important PILE OF POO (💩) character.

So, for the sake of meaningful and descriptive variable names, what gives? Does -fextended-identifiers do as it advertises or not? Is it only supported in the very latest build? And what kind of support do other compilers have?