Handle Windows not supporting static array dimension (aws#1912)

awslabs/s2n-bignum#98 handles Windows not supporting static array dimensions. We have our own header files with function declarations for s2n-bignum. Hence that work was never ported automatically.

Adopt same detection approach as in referenced PR. MSVC doesn't support it, neither does C++. So, try to detect those cases as well as if variable-length arrays aren't supported.