config.mak.uname: resolve FreeBSD iconv-related compilation warning

OLD_ICONV has long been needed by FreeBSD so config.mak.uname defines it unconditionally. However, recent versions do not need it, and its presence results in compilation warnings. Resolve this issue by defining OLD_ICONV only for older FreeBSD versions. Specifically, revision r281550[1], which is part of FreeBSD 11, removed the need for OLD_ICONV, and r282275[2] back-ported that change to 10.2. Versions prior to 10.2 do need it. [1] https://github.com/freebsd/freebsd/commit/b0813ee288f64f677a2cebf7815754b027a8215b [2] https://github.com/freebsd/freebsd/commit/b709ec868adb5170d09bc5a66b18d0e0d5987ab6 [es: commit message; tweak version check to distinguish 10.x versions] Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>