Old versions of libtool (1.5.10) used to set ECHO as a side effect. With
libtool 2.2.2 we need to do this manually

RP - 14/4/08

Index: libpng-1.2.16/Makefile.am
===================================================================
--- libpng-1.2.16.orig/Makefile.am	2008-04-14 14:47:21.000000000 +0100
+++ libpng-1.2.16/Makefile.am	2008-04-14 14:47:47.000000000 +0100
@@ -102,6 +102,8 @@
 			>$@.new
 	mv $@.new $@
 
+ECHO=echo
+
 libpng.vers: libpng.sym
 	rm -f $@ $@.new
 	$(ECHO) PNG@PNGLIB_MAJOR@@PNGLIB_MINOR@_0 '{global:' > $@.new