diff --git a/contrib/pngminus/png2pnm.c b/contrib/pngminus/png2pnm.c
index 879db0379..1420a783b 100644
--- a/contrib/pngminus/png2pnm.c
+++ b/contrib/pngminus/png2pnm.c
@@ -5,7 +5,8 @@
  *  version 1.0 - 1999.10.15 - First version.
  *          1.1 - 2017.04.22 - Add buffer-size check (Glenn Randers-Pehrson)
  *          1.2 - 2017.08.24 - Fix potential overflow in buffer-size check
-                               (Glenn Randers-Pehrson)
+ *                             (Glenn Randers-Pehrson)
+ *          1.3 - 2017.08.28 - Add PNGMINUS_UNUSED (Christian Hesse)
  *
  *  Permission to use, copy, modify, and distribute this software and
  *  its documentation for any purpose and without fee is hereby granted,
@@ -52,7 +53,13 @@
 #  define png_jmpbuf(png_ptr) ((png_ptr)->jmpbuf)
 #endif
 
-#define PNGMINUS_UNUSED(param) (void)param;
+#ifndef PNGMINUS_UNUSED
+/* Unused formal parameter warnings are silenced using the following macro
+ * which is expected to have no bad effects on performance (optimizing
+ * compilers will probably remove it entirely).
+ */
+#  define PNGMINUS_UNUSED(param) (void)param
+#endif
 
 /* function prototypes */
 
@@ -446,7 +453,7 @@ BOOL png2pnm (FILE *png_file, FILE *pnm_file, FILE *alpha_file,
   if (png_pixels != (unsigned char*) NULL)
     free (png_pixels);
 
-  PNGMINUS_UNUSED(raw) /* to quiet a Coverity defect */
+  PNGMINUS_UNUSED(raw); /* to quiet a Coverity defect */
   return TRUE;
 
 } /* end of source */
diff --git a/contrib/pngminus/pnm2png.c b/contrib/pngminus/pnm2png.c
index 18f5bed3d..229813666 100644
--- a/contrib/pngminus/pnm2png.c
+++ b/contrib/pngminus/pnm2png.c
@@ -7,6 +7,7 @@
  *  version 1.2 - 2017.04.22 - Add buffer-size check
  *          1.3 - 2017.08.24 - Fix potential overflow in buffer-size check
  *                             (Glenn Randers-Pehrson)
+ *          1.4 - 2017.08.28 - Add PNGMINUS_UNUSED (Christian Hesse)
  *
  *  Permission to use, copy, modify, and distribute this software and
  *  its documentation for any purpose and without fee is hereby granted,
@@ -50,7 +51,14 @@
 #  define png_jmpbuf(png_ptr) ((png_ptr)->jmpbuf)
 #endif
 
-#define PNGMINUS_UNUSED(param) (void)param;
+#ifndef PNGMINUS_UNUSED
+/* Unused formal parameter warnings are silenced using the following macro
+ * which is expected to have no bad effects on performance (optimizing
+ * compilers will probably remove it entirely).
+ */
+#  define PNGMINUS_UNUSED(param) (void)param
+#endif
+
 
 /* function prototypes */
 
@@ -515,7 +523,7 @@ BOOL pnm2png (FILE *pnm_file, FILE *png_file, FILE *alpha_file, BOOL interlace,
   if (png_pixels != (unsigned char*) NULL)
     free (png_pixels);
 
-  PNGMINUS_UNUSED(raw) /* Quiet a Coverity defect */
+  PNGMINUS_UNUSED(raw); /* Quiet a Coverity defect */
 
   return TRUE;
 } /* end of pnm2png */