Introduce conditional wchar_t (and std::wstring) support

The support is turned on by default but the user might need to be able
to turn it off which is now possible by defining CATCH_CONFIG_NO_WCHAR.

docs/configuration.md

@@ -120,6 +120,7 @@ by using `_NO_` in the macro, e.g. `CATCH_CONFIG_NO_CPP17_UNCAUGHT_EXCEPTIONS`.
     CATCH_CONFIG_WINDOWS_CRTDBG             // Enable leak checking using Windows's CRT Debug Heap
     CATCH_CONFIG_DISABLE_STRINGIFICATION    // Disable stringifying the original expression
     CATCH_CONFIG_DISABLE                    // Disables assertions and test case registration
+    CATCH_CONFIG_WCHAR                      // Enables use of wchart_t
 
 Currently Catch enables `CATCH_CONFIG_WINDOWS_SEH` only when compiled with MSVC, because some versions of MinGW do not have the necessary Win32 API support.
 
@@ -127,6 +128,9 @@ Currently Catch enables `CATCH_CONFIG_WINDOWS_SEH` only when compiled with MSVC,
 
 `CATCH_CONFIG_WINDOWS_CRTDBG` is off by default. If enabled, Windows's CRT is used to check for memory leaks, and displays them after the tests finish running.
 
+`CATCH_CONFIG_WCHAR` is on by default, but can be disabled. Currently
+it is only used in support for DJGPP cross-compiler.
+
 These toggles can be disabled by using `_NO_` form of the toggle, e.g. `CATCH_CONFIG_NO_WINDOWS_SEH`.
 
 ### `CATCH_CONFIG_FAST_COMPILE`