always use stdint types if available

The discussion came up here: https://github.com/DLR-SR/Noise/pull/24. Unfortunately this introduces new compiler warnings with Open WATCOM compiler where int32_t is long and not int. Let's hope Chris (from Matio) will address this issue (see https://sourceforge.net/p/matio/patches/8).

git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@7819 7ce873d0-865f-4ce7-a662-4bb36ea78beb