In the MPS, we have a rule that platform-specific feature tests go into ``mpstd.h``. See ["Target Platform Detection"](http://www.ravenbrook.com/project/mps/master/design/config/#pf) in the design docs.

In particular, if we are going to add Pelles C to the set of supported compilers, we should do it by defining a new platform code ``w3i3pc`` (Windows, IA-32, Pelles C) and setting it up in ``mpstd.h``. We'd also need to update the build system and the documentation accordingly.