[clang-fuzzer] Allow linking with any fuzzing engine.

[morehouse]

Summary

Makes clang-[proto-]fuzzer compatible with flags specified by OSS-Fuzz.

https://llvm.org/pr34314

[kcc]

LGTM

clang/tools/clang-fuzzer/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS ${LLVM_TARGETS_TO_BUILD} FuzzMutate)
 set(CXX_FLAGS_NOFUZZ ${CMAKE_CXX_FLAGS})
 set(DUMMY_MAIN DummyClangFuzzer.cpp)
-if(LLVM_USE_SANITIZE_COVERAGE)
+if(DEFINED LIB_FUZZING_ENGINE)
+  unset(DUMMY_MAIN)
+elseif(LLVM_USE_SANITIZE_COVERAGE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=fuzzer")
   set(CXX_FLAGS_NOFUZZ "${CXX_FLAGS_NOFUZZ} -fsanitize=fuzzer-no-link")
   unset(DUMMY_MAIN)
 endif()
 
 # Hack to bypass LLVM's cmake sources check and allow multiple libraries and
 # executables from this directory.
 set(LLVM_OPTIONAL_SOURCES
if(CLANG_ENABLE_PROTO_FUZZER)
   add_clang_executable(clang-proto-fuzzer
     ${DUMMY_MAIN}
     ExampleClangProtoFuzzer.cpp
     )
 
   target_link_libraries(clang-proto-fuzzer
     ${ProtobufMutator_LIBRARIES}
     ${PROTOBUF_LIBRARIES}
+    ${LIB_FUZZING_ENGINE}
     clangCXXProto
     clangHandleCXX
     clangProtoToCXX
     )
 endif()
 
 add_clang_subdirectory(handle-cxx)
 
 add_clang_executable(clang-fuzzer
   EXCLUDE_FROM_ALL
   ${DUMMY_MAIN}
   ClangFuzzer.cpp
   )
 
 target_link_libraries(clang-fuzzer
+  ${LIB_FUZZING_ENGINE}
   clangHandleCXX
   )