[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.hpgcc

@@ -25,6 +25,7 @@ AR_RC=ar rc
 MKDIR_P=mkdir -p
 LN_SF=ln -sf
 RANLIB=ranlib
+CP=cp
 RM_F=/bin/rm -f
 
 # where "make install" puts libpng.a, $(OLDSO)*, png.h, pngconf.h