Patch for adding -c to ranlib flags must not be applied when using llvm-ranlib

[climbfuji]

Trying to build develop or release 11.5.0 on macOS with LLVM clang + gfortran:

     846    /usr/local/opt/llvm/bin/llvm-ranlib -c libbufr_4_DA.a
  >> 847    /usr/local/opt/llvm/bin/llvm-ranlib: error: Invalid option: '-c'
  >> 848    make[2]: *** [src/libbufr_4_DA.a] Error 1
     849    make[2]: *** Deleting file `src/libbufr_4_DA.a'
  >> 850    make[1]: *** [src/CMakeFiles/bufr_4_DA.dir/all] Error 2
  >> 851    make: *** [all] Error 2

In CMakeLists.txt:

if(APPLE)
  # The linker on macOS does not include `common symbols` by default
  # Passing the -c flag includes them and fixes an error with undefined symbols
  set(CMAKE_Fortran_ARCHIVE_FINISH "<CMAKE_RANLIB> -c <TARGET>")
  set(CMAKE_C_ARCHIVE_FINISH "<CMAKE_RANLIB> -c <TARGET>")
endif()

I tested this with different clang versions. The native Apple clang requires the -c flag, otherwise the linker step fails with undefined symbols. The LLVM clang ranlib (llvm-ranlib) doesn't understand the -c flag and doesn't need it.