Bugfix/code clean using gcc warnings (#371)

* Enable extra GCC warnings to discover subtle bugs

* convert c++ comments to c comments

* cleanup pedantic compiler warnings

* Compile apps with GNU89 GNU99 GNU11 and GNU17

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

src/bacnet/bacstr.c

@@ -900,6 +900,8 @@ bool utf8_isvalid(const char *str, size_t length)
                     return false;
                 }
                 break;
+            default:
+                break;
         }
 
         /* Check for valid bytes after the 2nd, if any; all must start 10 */