When building in C mode, the VC runtime assumes that it can use pointer
aliasing through `char *` for the parameter to `__va_start`. Relax the
checks further. In theory we could keep the tests strict for non-system
header code, but this takes the less strict approach as the additional
check doesn't particularly end up being too much more helpful for
correctness. The C++ type system is a bit stricter and requires the
explicit cast which we continue to verify.
Saleem Abdulrasool