There are 2 variants of strerror_r, GNU extension and a POSIX variant.
Use the more portable POSIX variant.
Also fix a potensial stack-use-after-return bug, instead of relying on
undefined behavoir in GNU libc implementation. The strerror_r manpage
says:
> This may be either a pointer to a string that the function stores in
> buf, or a pointer to some (immutable) static string (in which case buf
> is unused).
So it does not guarantee that it returns a pointer to some (immutable)
static string. It might return a pointer to "buf", in which case we get
a stack-use-after-return bug.
We fix this by declaring a thread local static buffer which we always
return.
On systems, the buildprocess will complain:
[ 7%] Building CXX object src/CMakeFiles/conky.dir/c++wrap.cc.o
conky/src/c++wrap.cc: In function ‘std::string strerror_r(int)’:
conky/src/c++wrap.cc:66:37: warning: ignoring return value of ‘char* strerror_r(int, char*, size_t)’, declared with attribute warn_unused_result [-Wunused-result]
According to "man 3 strerror_r" the buffer used as 2nd arg is not always used, this patch also fixes that problem.
