libpng: properly fix zlib issue

The previous commit meant to avoid adding an LDFLAG but it just avoided
the issue. The real issue is PNG_BUILD_ZLIB being turned to ON. This
variable is for specifying a custom zlib location, which is not
necessary. Switching it to OFF calls find_package and links properly.

Backported two zlib related patches. The latter is a somewhat impoarant
fix.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/libpng/patches/030-pkgconfig-zlib.patch

@@ -0,0 +1,22 @@
+From dbe3e0c43e549a1602286144d94b0666549b18e6 Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Cristian=20Rodr=C3=ADguez?= <crrodriguez@opensuse.org>
+Date: Mon, 19 Mar 2018 12:41:47 -0300
+Subject: [PATCH] libpng.pc.in: zlib dependency is private
+
+zlib should be injected only when pkgconfig is ran with the --static
+option.
+---
+ libpng.pc.in | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/libpng.pc.in
++++ b/libpng.pc.in
+@@ -6,7 +6,7 @@ includedir=@includedir@/libpng@PNGLIB_MA
+ Name: libpng
+ Description: Loads and saves PNG files
+ Version: @PNGLIB_VERSION@
+-Requires: zlib
++Requires.private: zlib
+ Libs: -L${libdir} -lpng@PNGLIB_MAJOR@@PNGLIB_MINOR@
+ Libs.private: @LIBS@
+ Cflags: -I${includedir}