Fix compiler warnings by explicit casting in divsufsort.c

[rzikm]

On newer MSVC compilers, compilation of divsufsort.c causes C4242 warnings.

divsufsort.c(438): error C4242: 'function': conversion from '__int64' to 'int', possible loss of data

This and C4244 are similar warnings which should not be generally suppressed.

This PR fixes those warnings by explicitly casting ptrdiff_t to int via a helper macro.

The alternative is to change certain variable types, but the current approach seemed like less risky change.