BTW, the is a thinking issue. You have on Linux no dll - only lib's. dll's are Windows only.
Can it be, on windows the name is lpng.dll and under Linux the name is libpng.so ? If libpng.so is the same as lpng under windows you have to install this with the tools of your distribution like "apt-get install libpng-dev" for debian like systems. Then you must correct the names in the source and check if the entrypoints in the lib are realy the same.