conditionalize stdint.h inclusion on C99

[jeffhammond]

mpi-abi-stubs/mpi.h

Line 4 in 81c85f5

#include <stdint.h>

[dalcinl]

Conditionalize on MPI_ABI_{Aint|Offset} being defined? If these two are defined, then #include <stdint.h> is not needed.
Otherwise, what's the alternative? Just #error ?

[jeffhammond]

I don't know exactly what I want to do yet, but I'd like a helpful diagnostic message when the user compiles without support for intptr_t. I don't think it's hard. I just made the issue so I remember to solve it later.