add libpng as dependency

* NOTE: you will need to download the new required libraries

src/framework/CMakeLists.txt

@@ -87,6 +87,7 @@ FIND_PACKAGE(OpenAL REQUIRED)
 FIND_PACKAGE(VorbisFile REQUIRED)
 FIND_PACKAGE(Vorbis REQUIRED)
 FIND_PACKAGE(Ogg REQUIRED)
+FIND_PACKAGE(PNG REQUIRED)
 
 IF(CMAKE_BUILD_TYPE STREQUAL "Debug" OR CMAKE_BUILD_TYPE STREQUAL "RelWithDebInfo")
     MESSAGE(STATUS "Debug information: ON")
@@ -147,6 +148,7 @@ INCLUDE_DIRECTORIES(
     ${PHYSFS_INCLUDE_DIR}
     ${GMP_INCLUDE_DIR}
     ${ZLIB_INCLUDE_DIR}
+    ${PNG_INCLUDE_DIR}
     "${CMAKE_CURRENT_LIST_DIR}/.."
 )
 
@@ -159,6 +161,7 @@ SET(framework_LIBRARIES
     ${VORBISFILE_LIBRARY} ${VORBIS_LIBRARY} ${OGG_LIBRARY}
     ${OPENGL_LIBRARIES}
     ${OPENAL_LIBRARY}
+    ${PNG_LIBRARY}
     ${ADDITIONAL_LIBRARIES}
 )