message: Move Apple-specific build flags to Clang section
  This allows building FlatBuffers with gcc on macOS:
  avoid linking libc++ when not using clang
  look at compiler first, then OS-specific options
diff: @@ -220,10 +220,6 @@ if(EXISTS "${CMAKE_TOOLCHAIN_FILE}")
  # do not apply any global settings if the toolchain
  # is being configured externally
  message(STATUS "Using toolchain file: ${CMAKE_TOOLCHAIN_FILE}.")
  -elseif(APPLE)
  - set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -stdlib=libc++")
  - set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic -Werror -Wextra -Wno-unused-parameter")
  - set(FLATBUFFERS_PRIVATE_CXX_FLAGS "-Wold-style-cast")
  elseif(CMAKE_COMPILER_IS_GNUCXX)
  if(CYGWIN)
  set(CMAKE_CXX_FLAGS
  @@ -250,8 +246,13 @@ elseif(CMAKE_COMPILER_IS_GNUCXX)
  "${CMAKE_CXX_FLAGS} -fsigned-char")
  elseif(${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
  - set(CMAKE_CXX_FLAGS
  - "${CMAKE_CXX_FLAGS} -std=c++0x -Wall -pedantic -Werror -Wextra -Wno-unused-parameter")
  + if(APPLE)
  + set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
  + else()
  + set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
  + endif()
  +
  + set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic -Werror -Wextra -Wno-unused-parameter")
  set(FLATBUFFERS_PRIVATE_CXX_FLAGS "-Wold-style-cast")
  if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.8)
  list(APPEND FLATBUFFERS_PRIVATE_CXX_FLAGS "-Wimplicit-fallthrough" "-Wextra-semi" "-Werror=unused-private-field") # enable warning
  