FLEX_ARRAY: require platforms to support the C99 syntax

Before C99 syntax to express that the final member in a struct is an array of unknown number of elements, i.e., struct { ... T flexible_array[]; }; came along, GNU introduced their own extension to declare such a member with 0 size, i.e., T flexible_array[0]; and the compilers that did not understand even that were given a way to emulate it by wasting one element, i.e., T flexible_array[1]; As we are using more and more C99 language features, let's see if the platforms that still need to resort to the historical forms of flexible array member support are still there, by forcing all the flex array definitions to use the C99 syntax and see if anybody screams (in which case reverting the changes is rather easy). Signed-off-by: Junio C Hamano <gitster@pobox.com>