git-compat-util.h: auto-adjust to compiler support of FLEX_ARRAY a bit better

git-compat-util.h: auto-adjust to compiler support of FLEX_ARRAY a bit better

When declaring a structure with a flexible array member, instead
of defaulting to the c99 syntax for non-gnu compilers (which
burned people with older compilers), default to the traditional
and more portable "member[1]; /* more */" syntax.

At the same time, other c99 compilers should be able to take
advantage of the modern syntax to flexible array members without
being gcc.  Check __STDC_VERSION__ for that.

Signed-off-by: Junio C Hamano <gitster@pobox.com>