config.mak.dev: squelch -Wno-missing-braces for older gcc

Versions of gcc prior to 4.9 complain about an initialization like: struct inner { int x; }; struct outer { struct inner; }; struct outer foo = { 0 }; and insist on: struct outer foo = { { 0 } }; Newer compilers handle this just fine. And ignoring the window even on older compilers is fine; the resulting code is correct, but we just get caught by -Werror. Let's relax this for older compilers to make developer lives easier (we don't care much about non-developers on old compilers; they may see a warning, but it won't stop compilation). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>