Work over a bug in gnulib error()

The error() function from glibc correctly prefixes each message it
prints with program_name as set by set_program_name. However, its
replacement from gnulib, which is linked in on systems where this
function is not available, prints the name returned by getprogname()
instead. Due to this messages output by tar subprocess (which sets its
program name to 'tar (child)') become indiscernible from those printed
by the main process. In particular, this breaks the remfiles01.at and
remfiles02.at test cases.

* configure.ac: Define ENABLE_ERROR_PRINT_PROGNAME if using
gnulib error().
* src/tar.c [ENABLE_ERROR_PRINT_PROGNAME] (tar_print_progname): New function.
(main) [ENABLE_ERROR_PRINT_PROGNAME]: Set error_print_progname.

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