[libFuzzer] Don't use `#ifdef` for defined macros, instead use `#if`.

[kcc]

LGTM, thanks!
BTW, don't rush implementing this thing on Windows -- the code is too experimental for that.
(Do add stubs, if needed)

llvm/trunk/lib/Fuzzer/FuzzerShmemPosix.cpp

 //===- FuzzerShmemPosix.cpp - Posix shared memory ---------------*- C++ -* ===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 // SharedMemoryRegion
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
-#ifdef LIBFUZZER_POSIX
+#if LIBFUZZER_POSIX
 
 #include "FuzzerIO.h"
 #include "FuzzerShmem.h"
 
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <sys/mman.h>