The CASTOR/SHIFT function sstrerror_r() should call the "standard"
function strerror_r().  Before this modification the "standard"
strerror() function was being called which is not guranteed to be
thread safe.

Compiling the CASTOR/SHIFT function sstrerror_r() code as C++ code
as opposed to C code makes the GNU compiler check the return type
of the "standard" function strerror_r().

The return type of the "standard" function strerror_r() is char *
for the GNU version and int for the POSIX version.

The following two conditions must be true in order to use the
POSIX version of strerror_r():

1. _GNU_SOURCE must NOT be defined
2. Either _XOPEN_SOURCE should be defined as 600 or
   _POSIX_C_SOURCE should be defined as 200112L