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/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libpng
 PKG_VERSION:=1.6.37
-PKG_RELEASE:=9
+PKG_RELEASE:=10
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=@SF/libpng
@@ -35,7 +35,7 @@ endef
 
 CMAKE_OPTIONS += \
 	-DCMAKE_POSITION_INDEPENDENT_CODE=ON \
-	-DPNG_BUILD_ZLIB=ON \
+	-DPNG_BUILD_ZLIB=OFF \
 	-DPNG_SHARED=ON \
 	-DPNG_STATIC=ON \
 	-DPNG_EXECUTABLES=OFF \