add libfuzzer custom mutator, minor enhancements and fixes

custom_mutators/libfuzzer/FuzzerExtFunctions.h

@@ -0,0 +1,34 @@
+//===- FuzzerExtFunctions.h - Interface to external functions ---*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Defines an interface to (possibly optional) functions.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_EXT_FUNCTIONS_H
+#define LLVM_FUZZER_EXT_FUNCTIONS_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+namespace fuzzer {
+
+struct ExternalFunctions {
+  // Initialize function pointers. Functions that are not available will be set
+  // to nullptr.  Do not call this constructor  before ``main()`` has been
+  // entered.
+  ExternalFunctions();
+
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  RETURN_TYPE(*NAME) FUNC_SIG = nullptr
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+};
+} // namespace fuzzer
+
+#endif