In imath.h, replace stdint.h usage with c.h equivalents.
As things stood, buildfarm member dory failed. MSVC versions lacking
stdint.h are unusable for building PostgreSQL, but pg_config.h.win32
doesn't know that. Even so, we support other systems lacking stdint.h,
including buildfarm member gaur. Per a suggestion from Tom Lane.
Discussion: https://postgr.es/m/9598.
1550353336@sss.pgh.pa.us