How do I install a library in Arduino Yun cross compile environment?

Question

A few days ago, I've asked this question. Thanks to the replies I've received, I've set up a cross compile environment for Yun by following this guide.
I've created a virtual machine with virtualbox and installed Debian 8.0 on it. I installed all the software required to download and compile the OpenWRT-Attitude Adjustment version (which, I think, is the version used by Arduino Yun) on Debian.
So, I've downloaded the OpenWRT source code, downloaded its packages feed (with "git clone git://git.openwrt.org/12.09/packages.git"), downloaded and installed all available "feeds" (with: "./scripts/feeds update -a" "./scripts/feeds install -a") and then compiled it with: "make defconfig" "make prereq" and "make menuconfig".
When I typed in "make menuconfig", a graphical interface prompted. In that interface I selected:
Target System: (Atheros AR7xx/AR9xx);
Base System: I selected libc, libcc and marked as package libpthread and librt.
I left any other option as default. Then I compiled all with "time make V=99" command.
After about 2 hours, the compiling finished and I set the environment variables as the above mentioned guide described.
Looks like now I can compile any C code with this cross-compiler. Anyway, my application includes the png.h header in order to use the libpng library, and every time I try to cross-compile it, a "png.h not found" error pops up during the process.
So I tried to cross-compile the libpng library but when I ran "./configure" with the options for cross-compiling, it failed saying that zlib wasn't installed. So I downloaded the zlib library source code but looks like I can't cross-compile it.
Eventually I compiled zlib and libpng normally on Debian and now, when I try to cross-compile my application, it still says that it can't find png.h... So, how can I "integrate" the libpng library to the cross-compiling environment?
Sorry for this long question, but it is all I've done today.
As always, I thank you for any help you can give me.

• user31481: I'm voting to close this question as off-topic because it's more than 2 years old and have no answers.