[MLton] C99 runtime problems on Darwin

[Henry Cejtin]

Including platform.h at the top of every C file (at least before any
non-comments) and having it do the #defines first will definitely work.
The reason it has to be before any system includes is that they will have
#if's in them depending on the values.
I would definitely go for that, and also as Stephen suggests, defining the
`missing' constants there.  This way all the C code just codes to a nice
complete POSIX implementation and you convert (via the #defines) any
`incomplete' platform to a `complete' one.