pngminus: Use the system zlib by default

If a zlib source tree exists besides the libpng source tree in the
same parent directory, the pngminus build may fail, unless the zlib
tree is built fully. In order to avoid this failing scenario, do not
use the custom-built zlib by default.

(The custom-built zlib is still necessary on platforms that lack a
system-built zlib.)
diff --git a/contrib/pngminus/Makefile b/contrib/pngminus/Makefile
index 7eb503a..4bba079 100644
--- a/contrib/pngminus/Makefile
+++ b/contrib/pngminus/Makefile
@@ -11,9 +11,14 @@
 PNGLIB_SHARED = -L../.. -lpng
 PNGLIB_STATIC = ../../libpng.a
-ZINC = -I../../../zlib
-ZLIB_SHARED = -L../../../zlib -lz
-ZLIB_STATIC = ../../../zlib/libz.a
+# Uncomment the following if you have a custom zlib build at ../../../zlib
+#ZINC = -I../../../zlib
+#ZLIB_SHARED = -L../../../zlib -lz
+#ZLIB_STATIC = ../../../zlib/libz.a
+# Use the system zlib otherwise