static_wstring should be conditionally enabled

[jwt27]

Hi!

static_wstring fails to compile on platforms that don't have libc support
for wide-character strings. I got it to work by defining a no-op
std::swprintf(...), but obviously that isn't really a solution.

However, boost::config has feature detection macros for this: BOOST_NO_CWCHAR
and BOOST_NO_SWPRINTF. So ideally, static_wstring should be conditionally
enabled based on those.

[sdkrystian]

Reasonable, I'll take care of this soon

[alandefreitas]

Please note this was probably because of the standalone library. We would need two paths here.

[sdkrystian]

@alandefreitas I'm not sure if it's worth trying to replicate BOOST_NO_CWCHAR in standalone.. Might be better to just check for cwchar in standalone?

[alandefreitas]

Probably not. Whatever is easier in standalone. We can have a separate path for boost.