#381 4.9.0 experimental posix missing some unicode stuff

[Jim Michaels]

df.cpp:2376:17: error: 'wbuffer_convert' was not declared in this scope
df.cpp:2376:33: error: 'codecvt_utf8' was not declared in this scope
df.cpp:2376:46: error: expected primary-expression before 'char16_t'
df.cpp:2378:49: error: 'u16buffer' was not declared in this scope

all of these things were defined before 4.9.0. I was having problems with console output, so I decided to turn on my UNICODE iostream console output stuff.

I don't have a good unit test for this.

compiler is personal experimental 4.9.0 posix sjlj 64-bit.

[Alexx83]

Is it relevant for latest toolchains?