Index: lib/Fuzzer/CMakeLists.txt
===================================================================
--- lib/Fuzzer/CMakeLists.txt
+++ lib/Fuzzer/CMakeLists.txt
@@ -11,12 +11,17 @@
 
   add_subdirectory(Support)
 
+  if (${CMAKE_SYSTEM_NAME} MATCHES "Windows")
+    set(FUZZER_EXT_FUNCTIONS FuzzerExtFunctionsWeakAlias.cpp)
+  elseif (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+    set(FUZZER_EXT_FUNCTIONS FuzzerExtFunctionsWeak.cpp)
+  elseif (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+    set(FUZZER_EXT_FUNCTIONS FuzzerExtFunctionsDlsym.cpp)
+  endif()
+
   add_library(LLVMFuzzerNoMainObjects OBJECT
     FuzzerCrossOver.cpp
     FuzzerDriver.cpp
-    FuzzerExtFunctionsDlsym.cpp
-    FuzzerExtFunctionsWeak.cpp
-    FuzzerExtFunctionsWeakAlias.cpp
     FuzzerLoop.cpp
     FuzzerMerge.cpp
     FuzzerMutate.cpp
@@ -24,6 +29,7 @@
     FuzzerTracePC.cpp
     FuzzerTraceState.cpp
     FuzzerUtil.cpp
+    ${FUZZER_EXT_FUNCTIONS}
     )
   add_library(LLVMFuzzerNoMain STATIC
     $<TARGET_OBJECTS:LLVMFuzzerNoMainObjects>
Index: lib/Fuzzer/FuzzerExtFunctionsDlsym.cpp
===================================================================
--- lib/Fuzzer/FuzzerExtFunctionsDlsym.cpp
+++ lib/Fuzzer/FuzzerExtFunctionsDlsym.cpp
@@ -11,8 +11,6 @@
 // requires that clients of LibFuzzer pass ``--export-dynamic`` to the linker.
 // That is a complication we don't wish to expose to clients right now.
 //===----------------------------------------------------------------------===//
-#include "Support/Platform.h"
-#if LIBFUZZER_APPLE
 
 #include "FuzzerExtFunctions.h"
 #include "Support/UtilIO.h"
@@ -48,5 +46,3 @@
 }
 
 } // namespace fuzzer
-
-#endif // LIBFUZZER_APPLE
Index: lib/Fuzzer/FuzzerExtFunctionsWeak.cpp
===================================================================
--- lib/Fuzzer/FuzzerExtFunctionsWeak.cpp
+++ lib/Fuzzer/FuzzerExtFunctionsWeak.cpp
@@ -12,8 +12,6 @@
 // weak symbols to be undefined. That is a complication we don't want to expose
 // to clients right now.
 //===----------------------------------------------------------------------===//
-#include "Support/Platform.h"
-#if LIBFUZZER_LINUX
 
 #include "FuzzerExtFunctions.h"
 #include "Support/UtilIO.h"
@@ -49,5 +47,3 @@
 }
 
 } // namespace fuzzer
-
-#endif // LIBFUZZER_LINUX
Index: lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp
===================================================================
--- lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp
+++ lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp
@@ -8,8 +8,6 @@
 //===----------------------------------------------------------------------===//
 // Implementation using weak aliases. Works for Windows.
 //===----------------------------------------------------------------------===//
-#include "Support/Platform.h"
-#if LIBFUZZER_WINDOWS
 
 #include "FuzzerExtFunctions.h"
 #include "Support/UtilIO.h"
@@ -52,5 +50,3 @@
 }
 
 } // namespace fuzzer
-
-#endif // LIBFUZZER_WINDOWS
Index: lib/Fuzzer/Support/CMakeLists.txt
===================================================================
--- lib/Fuzzer/Support/CMakeLists.txt
+++ lib/Fuzzer/Support/CMakeLists.txt
@@ -1,10 +1,27 @@
-add_library(LLVMFuzzerSupport STATIC
+set(FUZZER_SUPPORT_SOURCES
   Util.cpp
-  UtilDarwin.cpp
-  UtilLinux.cpp
-  UtilPosix.cpp
-  UtilWindows.cpp
   UtilIO.cpp
-  UtilIOPosix.cpp
-  UtilIOWindows.cpp
   )
+
+if (${CMAKE_SYSTEM_NAME} MATCHES "Windows")
+  list(APPEND FUZZER_SUPPORT_SOURCES
+    UtilWindows.cpp
+    UtilIOWindows.cpp
+  )
+else()
+  list(APPEND FUZZER_SUPPORT_SOURCES
+    UtilPosix.cpp
+    UtilIOPosix.cpp
+  )
+  if (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+    list(APPEND FUZZER_SUPPORT_SOURCES
+      UtilLinux.cpp
+    )
+  elseif (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+    list(APPEND FUZZER_SUPPORT_SOURCES
+      UtilDarwin.cpp
+    )
+  endif()
+endif()
+
+add_library(LLVMFuzzerSupport STATIC ${FUZZER_SUPPORT_SOURCES})
Index: lib/Fuzzer/Support/UtilDarwin.cpp
===================================================================
--- lib/Fuzzer/Support/UtilDarwin.cpp
+++ lib/Fuzzer/Support/UtilDarwin.cpp
@@ -8,8 +8,6 @@
 //===----------------------------------------------------------------------===//
 // Misc utils for Darwin.
 //===----------------------------------------------------------------------===//
-#include "Platform.h"
-#if LIBFUZZER_APPLE
 
 #include "UtilIO.h"
 #include <mutex>
@@ -148,5 +146,3 @@
 }
 
 } // namespace fuzzer
-
-#endif // LIBFUZZER_APPLE
Index: lib/Fuzzer/Support/UtilIOPosix.cpp
===================================================================
--- lib/Fuzzer/Support/UtilIOPosix.cpp
+++ lib/Fuzzer/Support/UtilIOPosix.cpp
@@ -8,8 +8,6 @@
 //===----------------------------------------------------------------------===//
 // IO functions implementation using Posix API.
 //===----------------------------------------------------------------------===//
-#include "Platform.h"
-#if LIBFUZZER_POSIX
 
 #include "UtilIO.h"
 #include <cstdarg>
@@ -84,5 +82,3 @@
 }
 
 }  // namespace fuzzer
-
-#endif // LIBFUZZER_POSIX
Index: lib/Fuzzer/Support/UtilIOWindows.cpp
===================================================================
--- lib/Fuzzer/Support/UtilIOWindows.cpp
+++ lib/Fuzzer/Support/UtilIOWindows.cpp
@@ -8,8 +8,6 @@
 //===----------------------------------------------------------------------===//
 // IO functions implementation for Windows.
 //===----------------------------------------------------------------------===//
-#include "Platform.h"
-#if LIBFUZZER_WINDOWS
 
 #include "UtilIO.h"
 #include <cassert>
@@ -278,5 +276,3 @@
 }
 
 }  // namespace fuzzer
-
-#endif // LIBFUZZER_WINDOWS
Index: lib/Fuzzer/Support/UtilLinux.cpp
===================================================================
--- lib/Fuzzer/Support/UtilLinux.cpp
+++ lib/Fuzzer/Support/UtilLinux.cpp
@@ -8,8 +8,6 @@
 //===----------------------------------------------------------------------===//
 // Misc utils for Linux.
 //===----------------------------------------------------------------------===//
-#include "Platform.h"
-#if LIBFUZZER_LINUX
 
 #include <stdlib.h>
 #include <string>
@@ -21,5 +19,3 @@
 }
 
 } // namespace fuzzer
-
-#endif // LIBFUZZER_LINUX
Index: lib/Fuzzer/Support/UtilPosix.cpp
===================================================================
--- lib/Fuzzer/Support/UtilPosix.cpp
+++ lib/Fuzzer/Support/UtilPosix.cpp
@@ -9,7 +9,6 @@
 // Misc utils implementation using Posix API.
 //===----------------------------------------------------------------------===//
 #include "Platform.h"
-#if LIBFUZZER_POSIX
 #include "Util.h"
 #include "UtilIO.h"
 #include <cassert>
@@ -105,5 +104,3 @@
 }
 
 }  // namespace fuzzer
-
-#endif // LIBFUZZER_POSIX
Index: lib/Fuzzer/Support/UtilWindows.cpp
===================================================================
--- lib/Fuzzer/Support/UtilWindows.cpp
+++ lib/Fuzzer/Support/UtilWindows.cpp
@@ -8,8 +8,7 @@
 //===----------------------------------------------------------------------===//
 // Misc utils implementation for Windows.
 //===----------------------------------------------------------------------===//
-#include "Platform.h"
-#if LIBFUZZER_WINDOWS
+
 #include "Util.h"
 #include "UtilIO.h"
 #include <cassert>
@@ -185,5 +184,3 @@
 }
 
 } // namespace fuzzer
-
-#endif // LIBFUZZER_WINDOWS