[libpng15] Port recent changes from libpng-1.6.11beta

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.std

@@ -32,6 +32,7 @@ AR_RC = ar rc
 MKDIR_P = mkdir
 LN_SF = ln -sf
 RANLIB = ranlib
+CP = cp
 RM_F = rm -f
 AWK = awk
 SED = sed