Fixed unused-parameters/unused-variables warnings

Added a ARG_UNUSED macro in utils/magic.h, which
produces a simple cast to void to silence the warnings.
The reasons this is used as opposed to an attribute is
that most compilers should support this.
Besides this, the macro can be easily searched for in
the future to potentially remove (where possible) the
unused parameters.

plot/plotPNM.c

@@ -398,7 +398,7 @@ pnmTile (tile, cxp)
     if (IsSplit(tile))
     {
 	TileType dinfo;
-	int w, h, llx, lly, urx, ury;
+	int llx, lly, urx, ury;
 	Rect scaledClip;
 
         type = (SplitSide(tile)) ? SplitRightType(tile) : SplitLeftType(tile);
@@ -577,9 +577,8 @@ PlotPNM(fileName, scx, layers, xMask, width)
 					 */
 {
     FILE *fp = NULL;
-    Rect bbox;
     int bb_ysize, bb_xsize;
-    int i, x, y, tile_ydelta;
+    int x, y, tile_ydelta;
     int save_ds, iter;
     int scale_over_2, ds_over_2;
     float *strip = NULL;
@@ -1132,6 +1131,7 @@ PlotPNMTechLine(sectionName, argc, argv)
     int argc;			/* Number of arguments on line. */
     char *argv[];		/* Pointers to fields of line. */
 {
+    ARG_UNUSED(sectionName);
     int i, j, k, style;
     void PlotPNMSetDefaults();	/* Forward declaration */
 
@@ -1356,7 +1356,7 @@ PlotLoadStyles(filename)
     char fullName[256];
     char *buf;
     int newsec;
-    int ord, mask, color, outline, nfill, stipple;
+    int ord, mask, color, outline, stipple;
     int ir, ig, ib;
     char shortname;
     char longname[128];