config.mak.dev: disable suggest braces error on old clang versions

config.mak.dev: disable suggest braces error on old clang versions

Add the "-Wno-missing-braces" option when building with an old version
of clang to suppress the "suggest braces around initialization" error
in developer mode.

For example, using an old version of clang gives the following errors
(when in DEVELOPER=1 mode):

$ make builtin/merge-file.o
    CC builtin/merge-file.o
builtin/merge-file.c:29:23: error: suggest braces around initialization \
			    of subobject [-Werror,-Wmissing-braces]
        mmfile_t mmfs[3] = { 0 };
                             ^
                             {}
builtin/merge-file.c:31:20: error: suggest braces around initialization \
			    of subobject [-Werror,-Wmissing-braces]
        xmparam_t xmp = { 0 };
                          ^
                          {}
2 errors generated.

This example compiles without error/warning with updated versions of
clang.  Since this is an obsolete error, use the -Wno-missing-braces
option to silence the warning when using an older compiler.  This
avoids the need to update the code to use "{{0}}" style
initializations.

Upstream clang version 8 has the problem.  It was fixed in version 9.

The version of clang distributed by Apple with XCode has its own
unique set of version numbers.  Apple clang version 11 has the
problem.  It was fixed in version 12.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>