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>

config.mak.dev

@@ -59,9 +59,13 @@ endif
 
 # uninitialized warnings on gcc 4.9.2 in xdiff/xdiffi.c and config.c
 # not worth fixing since newer compilers correctly stop complaining
+#
+# Likewise, gcc older than 4.9 complains about initializing a
+# struct-within-a-struct using just "{ 0 }"
 ifneq ($(filter gcc4,$(COMPILER_FEATURES)),)
 ifeq ($(filter gcc5,$(COMPILER_FEATURES)),)
 DEVELOPER_CFLAGS += -Wno-uninitialized
+DEVELOPER_CFLAGS += -Wno-missing-braces
 endif
 endif