I'm a little confused by this issue. Looks like an empty flex array
is supported here: https://godbolt.org/z/j3ndYTEfx. 
(Note that I'm passing /TC to force C compilation).

Also, heap_fsentry doesn't appear to use a flex array in current sources.

If it does start using it, there issue may actually be elsewhere besides
the struct definition (it could be just a badly targeted compiler error).
We have code like `struct heap_fsentry key[2];`.  That declaration can't
work with a flex array.

Thanks,
Neeraj
(wearing the ex-MSVC dev hat)