Compile with wxWidgets 3.1.1 (on Windows).

3.1.1 includes hash<wxString> so we don't need to.
git now ignores the win-alt-build subdirectory, which can be used for an alternative set of
windows build files.  So for example, can keep an old 3.0.2 set up there whilst working on 3.1.1

.gitignore

@@ -182,6 +182,7 @@ win/Projects/*/Debug
 win/Projects/*/Release
 win/.vs/
 win/packages
+win-alt-build
 
 # All those help files
 help/manual*

src/MemoryX.h

@@ -1177,6 +1177,7 @@ namespace std
    namespace tr1
    {
 #endif
+#if !wxCHECK_VERSION(3, 1, 0)
       template<typename T> struct hash;
       template<> struct hash< wxString > {
          size_t operator () (const wxString &str) const // noexcept
@@ -1186,6 +1187,7 @@ namespace std
             return Hasher{}( stdstr );
          }
       };
+#endif
 #ifdef __AUDACITY_OLD_STD__
    }
 #endif