CodingGuidelines: mention C99 features we can't use

CodingGuidelines: mention C99 features we can't use

The C99 section of the CodingGuidelines is a good overview of what we
can use, but is sorely lacking in what we can't use. Something that
comes up occasionally is the portability of %z.

Per [1] we couldn't use it for the longest time due to MSVC not
supporting it, but nowadays by requiring C99 we rely on the MSVC
version that does, but we can't use it yet because a C library that
MinGW uses doesn't support it.

1. https://lore.kernel.org/git/a67e0fd8-4a14-16c9-9b57-3430440ef93c@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>