[libpng17] Port recent changes from libpng16 branch:

Silence 'unused parameter' build warnings (Cosmin).
  $(CP) is now used alongside $(RM_F).  Also, use 'copy' instead
    of 'cp' where applicable, and applied other minor makefile changes.
  Don't warn about invalid dimensions exceeding user limits.

scripts/makefile.gcc

@@ -16,6 +16,7 @@ CC = gcc
 LD = $(CC)
 AR_RC = ar rcs
 RANLIB = ranlib
+CP = cp
 RM_F = rm -f
 
 WARNMORE = -Wwrite-strings -Wpointer-arith -Wshadow \
@@ -39,7 +40,7 @@ all: static
 
 # see scripts/pnglibconf.mak for more options
 pnglibconf.h: scripts/pnglibconf.h.prebuilt
-	cp scripts/pnglibconf.h.prebuilt $@
+	$(CP) $< $@
 
 .c.o:
 	$(CC) -c $(CPPFLAGS) $(CFLAGS) -o $@ $<