If sync_file_range is not available when building the configure script,
there is a cosmetic bug when running that script reporting
"HAVE_SYNC_FILE_RANGE: command not found". Remove that error message by
defining HAVE_SYNC_FILE_RANGE to an empty string, rather than generating
a script where that appears as a bare command.
Signed-off-by: Adam Dinwoodie <adam@dinwoodie.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>