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

src/system_error.cpp

@@ -55,6 +55,7 @@ error_category::equivalent(const error_code& code, int condition) const _NOEXCEP
     return *this == code.category() && code.value() == condition;
 }
 
+#if !defined(_LIBCPP_HAS_NO_THREADS)
 namespace {
 
 //  GLIBC also uses 1024 as the maximum buffer size internally.
@@ -96,6 +97,7 @@ string do_strerror_r(int ev) {
 #endif
 
 } // end namespace
+#endif
 
 string
 __do_message::message(int ev) const