Fix GCC 4.9.3 Build Errors

Since GCC version 4.9.3 does not support the -Wimplicit-fallthrough warning option or the __attribute__((fallthrough)) annotation,
introduce the corresponding fixes to prevent build errors.

tinyxml2.cpp

@@ -446,17 +446,17 @@ void XMLUtil::ConvertUTF32ToUTF8( unsigned long input, char* output, int* length
             --output;
             *output = static_cast<char>((input | BYTE_MARK) & BYTE_MASK);
             input >>= 6;
-            //fall through
+            TIXML_FALLTHROUGH;
         case 3:
             --output;
             *output = static_cast<char>((input | BYTE_MARK) & BYTE_MASK);
             input >>= 6;
-            //fall through
+            TIXML_FALLTHROUGH;
         case 2:
             --output;
             *output = static_cast<char>((input | BYTE_MARK) & BYTE_MASK);
             input >>= 6;
-            //fall through
+            TIXML_FALLTHROUGH;
         case 1:
             --output;
             *output = static_cast<char>(input | FIRST_BYTE_MARK[*length]);