[WIP] Add <cuchar>

[Ed Smith-Rowland]

This patch is attempting to add the C++11 header cuchar that wraps the C11 uchar.h header that provides means of conversion between char16_t, char32_t and multibyte character sequences.  Recent glib versions ship with this support and it would be nice to forward this in C++11.

It's not ready but I wanted to get some feedback largely on configuration and autohell issues.  Also, in the case where there is no support on the system for uchar.h, what should we do?  I added a #warning that the capability is not supported and left the guts of cuchar #if-d out.  Honestly, my goal was to check off a low-priority box and lean some things.

Thanks,

Ed

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