make check for alternate linke more restrictive: also check for linker binary.

        OR (${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang"))

      include(CheckCXXCompilerFlag)

      set(CMAKE_CUSTOM_LINKER_DEFAULT default)

      check_cxx_compiler_flag(-fuse-ld=lld HAVE_LLD_LINKER)

      check_cxx_compiler_flag(-fuse-ld=gold HAVE_GOLD_LINKER)

      check_cxx_compiler_flag(-fuse-ld=bfd HAVE_BFD_LINKER)

      if(HAVE_LLD_LINKER)

      check_cxx_compiler_flag(-fuse-ld=lld HAVE_LLD_LINKER_FLAG)

      check_cxx_compiler_flag(-fuse-ld=gold HAVE_GOLD_LINKER_FLAG)

      check_cxx_compiler_flag(-fuse-ld=bfd HAVE_BFD_LINKER_FLAG)

      find_program(HAVE_LLD_LINKER_BIN lld ld.lld)

      find_program(HAVE_GOLD_LINKER_BIN ld.gold)

      find_program(HAVE_BFD_LINKER_BIN ld.bfd)

      if(HAVE_LLD_LINKER_FLAG AND HAVE_LLD_LINKER_BIN)

        set(CMAKE_CUSTOM_LINKER_DEFAULT lld)

      elseif(HAVE_GOLD_LINKER)

      elseif(HAVE_GOLD_LINKER_FLAG AND HAVE_GOLD_LINKER_BIN)

        set(CMAKE_CUSTOM_LINKER_DEFAULT gold)

      elseif(HAVE_BFD_LINKER)

      elseif(HAVE_BFD_LINKER_FLAG AND HAVE_BFD_LINKER_BIN)

        set(CMAKE_CUSTOM_LINKER_DEFAULT bfd)

      endif()

      set(CMAKE_CUSTOM_LINKER_VALUES lld gold bfd default)