We require C++11 now, simplify this code.

This ensures wstring to std::string conversion is always available,
needed for some HID work I’m doing on a branch,.

simgear/misc/strutils.cxx

@@ -29,10 +29,7 @@
 #include <cstring>              // strerror_r() and strerror_s()
 #include <cctype>
 #include <cerrno>
-
-#if defined(HAVE_CPP11_CODECVT)
-    #include <codecvt> // new in C++11
-#endif
+#include <codecvt>
  
 #include "strutils.hxx"
 
@@ -656,26 +653,14 @@ static std::string convertWStringToMultiByte(DWORD encoding, const std::wstring&
 
 std::wstring convertUtf8ToWString(const std::string& a)
 {
-#ifdef SG_WINDOWS
-    return convertMultiByteToWString(CP_UTF8, a);
-#elif defined(HAVE_CPP11_CODECVT)
     std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t> ucs2conv;
     return ucs2conv.from_bytes(a);
-#else
-    return std::wstring();
-#endif
 }
 
 std::string convertWStringToUtf8(const std::wstring& w)
 {
-#ifdef SG_WINDOWS
-	return convertWStringToMultiByte(CP_UTF8, w);
-#elif defined(HAVE_CPP11_CODECVT)
     std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t> ucs2conv;
     return ucs2conv.to_bytes(w);
-#else
-    return std::string();
-#endif
 }
 
 std::string convertWindowsLocal8BitToUtf8(const std::string& a)