system_error: use strerror_r only for threaded code

When building libc++ without threading, strerror_r is not used.  Define
the code only when threading is enabled.  This allows us to build
system_error for Windows, which ATM doesn't build with threading.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290791 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed