small: fix small compilation
small: fix small compilation

If SMALL_EMBEDDED is used, "small" will try to compile
with the default flags, but it can't be compiled with std=c89.
Look like a "small" library bug.
So, we will use std=c99 globally as workaround.

See https://github.com/tarantool/small/issues/25

Part of #59

@Totktonada: added the comment.

tarantool/memcached,tarantool/memcached