Add helper macro to handle printf .* conversion for 32/64

Field width conversion `.*` always takes an `int`.

On Win32, casting `long` to `int` triggers a `-Wuseless-cast` warning.
Omitting the cast works correctly.

On Linux/amd64, casting `long` to `int` works correctly.
Omitting the cast triggers a `-Wformat` warning.

Add a macro that conditionally expands to `static_cast<int>` or to ``,
as necessary for the target platform.

SConstruct

@@ -1875,6 +1875,32 @@ $ x86_64-pc-linux-gnu-g++-5.4.0 -x c++ -S -Wformat -o /dev/null -
 				return
 		raise SCons.Errors.StopError("C++ compiler rejects all candidate format strings for std::size_t.")
 	@_custom_test
+	def check_compiler_useless_cast(self,context):
+		context.sconf.Define('DXX_ptrdiff_cast_int', 'static_cast<int>'
+			if self.Compile(context, text='''
+#include <cstdio>
+''', main='''
+	char a[8];
+	snprintf(a, sizeof(a), "%.*s", static_cast<int>(&argv[0][1] - &argv[0][0]), "0");
+''', msg='whether to cast operator-(T*,T*) to int')
+			else '',
+		'''
+For mingw32-gcc-5.4.0 and x86_64-pc-linux-gnu-gcc-5.4.0, gcc defines
+`long operator-(T *, T *)`.
+
+For mingw32, gcc reports a useless cast converting `long` to `int`.
+For x86_64-pc-linux-gnu, gcc does not report a useless cast converting
+`long` to `int.
+
+Various parts of the code take the difference of two pointers, then cast
+it to `int` to be passed as a parameter to `snprintf` field width
+conversion `.*`.  The field width conversion is defined to take `int`,
+so the cast is necessary to avoid a warning from `-Wformat` on
+x86_64-pc-linux-gnu, but is not necessary on mingw32.  However, the cast
+causes a -Wuseless-cast warning on mingw32.
+'''
+		)
+	@_custom_test
 	def check_strcasecmp_present(self,context,_successflags={'CPPDEFINES' : ['DXX_HAVE_STRCASECMP']}):
 		main = '''
 	return !strcasecmp(argv[0], argv[0] + 1) && !strncasecmp(argv[0] + 1, argv[0], 1);

d2x-rebirth/libmve/decoder16.cpp

@@ -77,8 +77,10 @@ void decodeFrame16(unsigned char *pFrame, const unsigned char *pMap, int mapRema
         FramePtr += 7*g_width;
     }
 
-    if ((length-(pData-pOrig)) != 0) {
-    	con_printf(CON_CRITICAL, "DEBUG: junk left over: %d,%d,%d", static_cast<int>(pData-pOrig), length, static_cast<int>(length-(pData-pOrig)));
+	const std::ptrdiff_t remaining = (pData - pOrig);
+	if (const std::ptrdiff_t difference = length - remaining)
+	{
+		con_printf(CON_CRITICAL, "DEBUG: junk left over: %d,%d,%d", DXX_ptrdiff_cast_int(remaining), length, DXX_ptrdiff_cast_int(difference));
     }
 }
 

d2x-rebirth/main/bmread.cpp

@@ -240,7 +240,7 @@ static void ab_load(int skip, const char * filename, array<bitmap_index, MAX_BIT
 	d_splitpath( filename, &path);
 	
 	for (i=0; i<MAX_BITMAPS_PER_BRUSH; i++ )	{
-		snprintf( tempname, sizeof(tempname), "%.*s#%d", static_cast<int>(path.base_end - path.base_start), path.base_start, i );
+		snprintf( tempname, sizeof(tempname), "%.*s#%d", DXX_ptrdiff_cast_int(path.base_end - path.base_start), path.base_start, i );
 		bi = piggy_find_bitmap( tempname );
 		if ( !bi.index )	
 			break;
@@ -262,7 +262,7 @@ static void ab_load(int skip, const char * filename, array<bitmap_index, MAX_BIT
 	}
 
 	for (i=0;i< *nframes; i++)	{
-		snprintf( tempname, sizeof(tempname), "%.*s#%d", static_cast<int>(path.base_end - path.base_start), path.base_start, i );
+		snprintf( tempname, sizeof(tempname), "%.*s#%d", DXX_ptrdiff_cast_int(path.base_end - path.base_start), path.base_start, i );
 		gr_remap_bitmap_good(*bm[i].get(), newpal, iff_has_transparency ? iff_transparent_color : -1, SuperX);
 
 		bm[i]->avg_color = compute_average_pixel(bm[i].get());

d2x-rebirth/main/gamepal.cpp

@@ -69,7 +69,7 @@ int load_palette(const char *name,int used_for_level,int no_change_screen)
 
 		struct splitpath_t path;
 		d_splitpath(name,&path);
-		snprintf(pigname, sizeof(pigname), "%.*s.pig", static_cast<int>(path.base_end - path.base_start), path.base_start);
+		snprintf(pigname, sizeof(pigname), "%.*s.pig", DXX_ptrdiff_cast_int(path.base_end - path.base_start), path.base_start);
 		//if not editor, load pig first so small install message can come
 		//up in old palette.  If editor version, we must load the pig after
 		//the palette is loaded so we can remap new textures.

similar/editor/kmine.cpp

@@ -189,7 +189,7 @@ static int med_save_situation(char * filename)
 	//	Write mine name.
 	struct splitpath_t path;
 	d_splitpath(filename, &path);
-	PHYSFSX_printf(SaveFile, "%.*s.min\n", static_cast<int>(path.base_end - path.base_start), path.base_start);
+	PHYSFSX_printf(SaveFile, "%.*s.min\n", DXX_ptrdiff_cast_int(path.base_end - path.base_start), path.base_start);
 
 	//	Write player position.
         PHYSFSX_printf(SaveFile, "%x %x %x\n",(unsigned int) ConsoleObject->pos.x,(unsigned int) ConsoleObject->pos.y,(unsigned int) ConsoleObject->pos.z);

similar/main/gamesave.cpp

@@ -1223,7 +1223,7 @@ int load_level(const char * filename_passed)
 	auto LoadFile = PHYSFSX_openReadBuffered(filename);
 	if (!LoadFile)
 	{
-		snprintf(filename, sizeof(filename), "%.*s%s", static_cast<int>(std::distance(Current_mission->path.cbegin(), Current_mission->filename)), Current_mission->path.c_str(), filename_passed);
+		snprintf(filename, sizeof(filename), "%.*s%s", DXX_ptrdiff_cast_int(std::distance(Current_mission->path.cbegin(), Current_mission->filename)), Current_mission->path.c_str(), filename_passed);
 		LoadFile = PHYSFSX_openReadBuffered(filename);
 	}
 

similar/main/piggy.cpp

@@ -359,7 +359,7 @@ bitmap_index piggy_find_bitmap(const char * name)
 			if (t) {                //extra stuff for ABMs
 				struct splitpath_t path;
 				d_splitpath(i.file_name, &path);
-				snprintf(temp, sizeof(temp), "%.*s%s\n", static_cast<int>(path.base_end - path.base_start), path.base_start, t);
+				snprintf(temp, sizeof(temp), "%.*s%s\n", DXX_ptrdiff_cast_int(path.base_end - path.base_start), path.base_start, t);
 				name = temp;
 			}
 			else