[MLton] C_Int_t format string

[Stephen Weeks]

When compiling the runtime on Cygwin, I get the following warnings

  basis/Real/gdtoa.c:38: warning: int format, C_Int_t arg (arg 5)
  basis/Real/gdtoa.c:38: warning: int format, C_Int_t arg (arg 6)
  basis/Real/gdtoa.c:72: warning: int format, C_Int_t arg (arg 5)
  basis/Real/gdtoa.c:72: warning: int format, C_Int_t arg (arg 6)

What's the best way to fix these?