Avoid some C compiler warnings on Cygwin.

library/int.m:
	Avoid warnings about overflows in implicit constant conversions in the
	implementations of {min,max}_int/0 on Cygwin.  The C compiler is
	complaining about where we assign LONG_MIN or LLONG_MIN to a 32-bit
	MR_Integer.  This only occurs in code branches that are dead when
	MR_Integer is 32-bit, so the fix here is just to put in some explicit
	casts.