[PATCH] Fix libstdc++/7811

[Roland McGrath]

> I'm unsure about the following points: in case LC_ALL is not set is the 
> fallback to LANG  really needed (PR submitter implicitly expected this 
> to be the case)? Also, if neither are set is always correct that "" -> "C"?

The standards specify looking at LC_ALL, then at the variable for the 
particular category if setting just one, then at LANG.  Note that a variable
that is present but empty (e.g. getenv("LC_ALL") => "") is treated as not
present.  Off hand I am not sure if the behavior when none are set is
specified or is implementation-defined.  Defaulting to "C" is safe and
consistent.

Another thing to note if you want to match the selection behavior of
setlocale is that if the name contains a slash and the program is setuid
then the name will be ignored and "C" used instead.  (It doesn't check
this on each variable, just on the name chosen from any.)