Issue #10910: Avoid C++ compilation errors on FreeBSD and OS X.

Patch by Ronald Oussoren.

Include/pyport.h

@@ -713,6 +713,12 @@ extern int fdatasync(int);
 #endif
 
 #ifdef _PY_PORT_CTYPE_UTF8_ISSUE
+#ifndef __cplusplus
+   /* The workaround below is unsafe in C++ because
+    * the <locale> defines these symbols as real functions,
+    * with a slightly different signature.
+    * See issue #10910
+    */
 #include <ctype.h>
 #include <wctype.h>
 #undef isalnum
@@ -730,6 +736,7 @@ extern int fdatasync(int);
 #undef toupper
 #define toupper(c) towupper(btowc(c))
 #endif
+#endif
 
 
 /* Declarations for symbol visibility.