Introduce UTF_CPP_CPLUSPLUS macro.

Let the users set the C++ standard version they want to support.

README.md

@@ -94,7 +94,11 @@ With a more modern compiler, the same operation would look like:
 ```
 If `__cplusplus` macro points to a C++ 11 or later, the library exposes API that takes into 
 account C++ standard Unicode strings and move semantics. With an older compiler, it is still
-possible to use the same functionality, just in a little less convenient way.
+possible to use the same functionality, just in a little less convenient way
+
+In case you do not trust the `__cplusplus` macro or, for instance, do not want to include
+the C++ 11 helper functions even with a modern compiler, define `UTF_CPP_CPLUSPLUS` macro
+before including `utf8.h` and assign it a value for the standard you want to use - the values are the same as for the `__cplusplus` macro. This can be also useful with compilers that are conservative in setting the `__cplusplus` macro even if they have a good support for a recent standard edition - Microsoft's Visual C++ is one example.
 
 ### Checking if a file contains valid UTF-8 text
 

source/utf8.h

@@ -31,7 +31,15 @@ DEALINGS IN THE SOFTWARE.
 #include "utf8/checked.h"
 #include "utf8/unchecked.h"
 
-#if __cplusplus >= 201103L // C++ 11 or later
+// Determine whether to include C++ 11 specific header
+// If the user defines UTF_CPP_CPLUSPLUS, use that.
+// Otherwise, trust the unreliable predefined macro __cplusplus
+
+#if !defined UTF_CPP_CPLUSPLUS
+    #define UTF_CPP_CPLUSPLUS __cplusplus
+#endif
+
+#if UTF_CPP_CPLUSPLUS >= 201103L // C++ 11 or later
 #include "utf8/cpp11.h"
 #endif // C++ 11 or later