autoconf: Add test for OLD_ICONV (squelching compiler warning)
Update configure.ac (and config.mak.in) to keep up with git
development by adding [compile] test whether your library has an old
iconv(), where the second (input buffer pointer) parameter is declared
with type (const char **) (OLD_ICONV).
Test-proposed-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>