Removed support of pngprefs.h from makefiles.

It is no longer used since the MMX support was removed.
diff --git a/scripts/descrip.mms b/scripts/descrip.mms
index bb38aac..30fb1cf 100644
--- a/scripts/descrip.mms
+++ b/scripts/descrip.mms
@@ -1,6 +1,6 @@
 
 cc_defs = /inc=$(ZLIBSRC)
-c_deb = 
+c_deb =
 
 .ifdef __DECC__
 pref = /prefix=all
@@ -15,12 +15,9 @@
 
 CFLAGS= $(C_DEB) $(CC_DEFS) $(PREF)
 
-all : pngtest.exe libpng.olb pngdefs.h
+all : pngtest.exe libpng.olb
 		@ write sys$output " pngtest available"
 
-pngdefs.h:
-	echo "/* pngdefs.h was built by MMS */" > pngdefs.h
-
 libpng.olb : libpng.olb($(OBJS))
 	@ write sys$output " Libpng available"
 
@@ -32,23 +29,24 @@
    run pngtest
 
 clean :
-	delete *.obj;*,*.exe;*,pngdefs.h
+	delete *.obj;*,*.exe;
 
 
 # Other dependencies.
-png.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngpread.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngset.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngget.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngread.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngrtran.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngrutil.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngerror.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngmem.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngrio.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngwio.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngtrans.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngwrite.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngwtran.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngwutil.obj : png.h, pngconf.h, pngdefs.h, pngpriv.h
-pngtest.obj : png.h, pngconf.h, pngdefs.h
+png.obj : png.h, pngconf.h, pngpriv.h
+pngpread.obj : png.h, pngconf.h, pngpriv.h
+pngset.obj : png.h, pngconf.h, pngpriv.h
+pngget.obj : png.h, pngconf.h, pngpriv.h
+pngread.obj : png.h, pngconf.h, pngpriv.h
+pngrtran.obj : png.h, pngconf.h, pngpriv.h
+pngrutil.obj : png.h, pngconf.h, pngpriv.h
+pngerror.obj : png.h, pngconf.h, pngpriv.h
+pngmem.obj : png.h, pngconf.h, pngpriv.h
+pngrio.obj : png.h, pngconf.h, pngpriv.h
+pngwio.obj : png.h, pngconf.h, pngpriv.h
+pngtrans.obj : png.h, pngconf.h, pngpriv.h
+pngwrite.obj : png.h, pngconf.h, pngpriv.h
+pngwtran.obj : png.h, pngconf.h, pngpriv.h
+pngwutil.obj : png.h, pngconf.h, pngpriv.h
+
+pngtest.obj : png.h, pngconf.h, pngpriv.h