[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.tc3

@@ -12,6 +12,8 @@ CC=tcc
 LD=tcc
 LIB=tlib
 LDFLAGS=-m$(MODEL) -L..\zlib
+CP=copy
+
 O=.obj
 E=.exe
 
@@ -27,7 +29,7 @@ all: libpng$(MODEL).lib pngtest$(E)
 
 # see scripts/pnglibconf.mak for more options
 pnglibconf.h: scripts/pnglibconf.h.prebuilt
-	cp scripts/pnglibconf.h.prebuilt $@
+	$(CP) $< $@
 
 pngtest: pngtest$(E)