Improved support for Pelles C:
* Refactor nmake files so that a compiler-specific makefile is included; move Microsoft Visual C-specific options to mv.nmk.
* Add nmake files for Pelles (w3i3pc.nmk and pc.nmk).
* Rename spw3i3mv.c to spw3i3.c and spw3i6mv.c to spw3i6.c since these are also used by Pelles C.
* Make reasonable changes to the source code to avoid warnings from Pelles C:
** check results of function calls;
** avoid useless return values;
** undef max before defining it;
** ensure printf formats are checkable;
** move notreached() assertions to the end of blocks;
** suppress warnings in cases where the code shouldn't be changed ("Unreachable code", "Inline assembly code is not portable", "Structured Exception Handling is not portable").