system_error: use strerror_r only for threaded code
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: 756ef344af921d95d562d9e9f9389127a89a6314@290791 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx