[Fuzzer] Avoid C++ compiler checks for fuzzer C++ library

[phosek]

Summary

When bulding libFuzzer as part of the toolchain, C++ library may not
have been fully built yet so the C++ compiler checks will fail, but we
don't care since we don't need C++ library anyway as we're building
our own.

compiler-rt/trunk/lib/fuzzer/CMakeLists.txt

if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND COMPILER_RT_LIBCXX_PATH)
   foreach(arch ${FUZZER_SUPPORTED_ARCH})
     get_target_flags_for_arch(${arch} TARGET_CFLAGS)
     set(LIBCXX_${arch}_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/libcxx_fuzzer_${arch})
     add_custom_libcxx(libcxx_fuzzer_${arch} ${LIBCXX_${arch}_PREFIX}
       CFLAGS ${TARGET_CFLAGS}
              -D_LIBCPP_ABI_VERSION=Fuzzer
              -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS=1
              -fvisibility=hidden
-      CMAKE_ARGS -DLIBCXX_ENABLE_EXCEPTIONS=OFF
+      CMAKE_ARGS -DCMAKE_CXX_COMPILER_WORKS=ON
+                 -DLIBCXX_ENABLE_EXCEPTIONS=OFF
                  -DLIBCXX_CXX_ABI=none)
     target_compile_options(RTfuzzer.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer.${arch} libcxx_fuzzer_${arch}-build)
     target_compile_options(RTfuzzer_main.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer_main.${arch} libcxx_fuzzer_${arch}-build)
     partially_link_libcxx(fuzzer_no_main ${LIBCXX_${arch}_PREFIX} ${arch})
     partially_link_libcxx(fuzzer ${LIBCXX_${arch}_PREFIX} ${arch})
   endforeach()