* symfile.c: Define two new hooks for symbol reading: "pre_add_symbol_hook"

    * symfile.c: Define two new hooks for symbol reading: "pre_add_symbol_hook"
        and "post_add_symbol_hook". These hooks are called before we begin reading
        symbols, and after we finish.
        (generic_load): Use new symbol reading hooks and get rid of compiler warning.
        * gdbtk.c (gdbtk_init): Add hooks for pre- and post-symbol reading.
        (gdbtk_pre_add_symbol): New function: the pre-add-symbol hook.
        (gdbtk_post_add_symbol): New function: the post-add-symbol hook.
        (find_file_in_dir): New function. Moved the guts of gdb_find_file_command
        into here to allow its use by others.
        (gdb_loc): Use find_file_in_dir to return the real path to the file
        (or "N/A" if we can't find it).
        * configure.in (TIX_LIB_EXT): Define new variable for those special cases
        when TCL_SHLIB_SUFFIX is not enough to specify the dependency.
        * configure: Regenerate.