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/plotHP.c

@@ -299,7 +299,7 @@ PlotDumpHPRTL(hpfile, kRaster, cRaster, mRaster, yRaster)
     Raster *mRaster;
     Raster *yRaster;
 {
-    int line, count, line_offset = 0;
+    int line, count;
     int ipl, bpl;
     register int *c, *m, *y, *k;
     unsigned char *obytes;	/* bytes to output (compressed) */