Relax warnings as errors so we can build on Clang 11 (#90)

This prevents TiKV building on MacOS. See also https://github.com/pingcap/rocksdb/pull/123Signed-off-by: Nick Cameron <nrc@ncameron.org>

cmake/rocksdb_flags.cmake

@@ -143,8 +143,13 @@ option(FAIL_ON_WARNINGS "Treat compile warnings as errors" ON)
 if(FAIL_ON_WARNINGS)
   if(MSVC)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /WX")
-  else() # assume GCC
+  else()
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
+    if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+      if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 11)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=shadow -Wno-error=defaulted-function-deleted")
+      endif()
+    endif()
   endif()
 endif()