`get_rootdir'
-------------

get_rootdir()
     :: Gets the name of Asir root directory.

RETURN
     string

   * On UNIX it returns the value of an environment variable
     `ASIR_LIBDIR' or `/usr/local/lib/asir' if `ASIR_LIBDIR' is not set.

   * On Windows the name of Asir root directory is returned.

   * By using relative path names from the value of this function, one
     can write programs which contain file operations independent of
     the install directory.

