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/plotPS.c

@@ -111,7 +111,6 @@ static PSStyle *curStyle;	/* Current style being output. */
 static PSColor *curColor;	/* Current color being output. */
 static PSPattern *curPattern;	/* Current pattern being output. */
 static int curLineWidth;	/* Current line width */
-static int curFont;		/* Current font */
 static TileTypeBitMask curMask;	/* Layers currently being searched:  this
 				 * is the AND of the mask from curStyle and
 				 * the layers that the user specified.
@@ -174,7 +173,6 @@ PSReset()
 void
 PlotPSTechInit()
 {
-    int i, j;
     PSStyle *style;
     PSColor *color;
     PSPattern *pattern;
@@ -230,6 +228,7 @@ PlotPSTechLine(sectionName, argc, argv)
     int argc;			/* Number of arguments on line. */
     char *argv[];		/* Pointers to fields of line. */
 {
+    ARG_UNUSED(sectionName);
     PSStyle *newstyle;
     PSColor *newcolor;
     PSPattern *newpattern;
@@ -382,7 +381,7 @@ plotPSLine(p1, p2)
 				 * coordinates.
 				 */
 {
-    int x1, x2, y1, y2, limit, diff;
+    int x1, x2, y1, y2, limit;
     bool tmptf;
 
     /* Clip the line to the rectangular area being output.  First,
@@ -525,7 +524,6 @@ plotPSPaint(tile, cxp)
 	 * to corner, and skip the rest of this procedure.
 	 */
 
-	Point ul, lr;
 
 	if (curLineWidth != PS_MEDIUM) {
 	    fprintf(file, "l2\n");
@@ -1135,7 +1133,7 @@ PlotPS(fileName, scx, layers, xMask)
 {
     int xsize, ysize;
     float yscale;
-    int i, j;
+    int j;
     int twidth, theight;
     char *fontptr, *fptr2, *fptr3;
     char line_in[100];