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

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>