flex-array: simplify compiler-specific workaround

We use "type array[];" syntax for the flex-array member at the end of a struct under C99 or later, except when we are building with older SUNPRO_C compilers. As we find more vendor compilers that claim to grok C99 but not understand the flex-array syntax, the existing "If we are using C99, but not with these compilers..." conditional will keep growing. Make it more manageable by listing vendor-specific exceptions earlier, with the expectation that new exceptions will not be combined into existing ones to make the condition longer, and instead will be implemented as a new "#elif" in the cascade of similar to old SUNPRO_C, we can just add a single line #elif defined(_MSC_VER) immediately before "#elif defined(__GNUC__)" to cause us to fallback to the safer but a bit wasteful version. Signed-off-by: Junio C Hamano <gitster@pobox.com>