CMake: Remove useless dependency on libpng

Libpng is a dependency of SDL2_image, not a direct dependency, so we
don't need to link against it.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

CMakeLists.txt

@@ -146,7 +146,6 @@ endif (USE_SYSTEM_LIBLUA)
 find_package(Freetype REQUIRED)
 find_package(OpenGL REQUIRED)
 find_package(PkgConfig REQUIRED)
-find_package(PNG REQUIRED)
 
 pkg_check_modules(SDL2 REQUIRED sdl2)
 pkg_check_modules(SDL2_IMAGE REQUIRED SDL2_image)
@@ -161,7 +160,6 @@ include_directories(
 	${ASSIMP_INCLUDE_DIRS}
 	${FREETYPE_INCLUDE_DIRS}
 	${OPENGL_INCLUDE_DIRS}
-	${PNG_INCLUDE_DIRS}
 	${SDL2_INCLUDE_DIRS}
 	${SDL2_IMAGE_INCLUDE_DIRS}
 	${SIGCPP_INCLUDE_DIRS}
@@ -226,7 +224,6 @@ list(APPEND pioneerLibs
 	${ASSIMP_LIBRARIES}
 	${FREETYPE_LIBRARIES}
 	${OPENGL_LIBRARIES}
-	${PNG_LIBRARIES}
 	${SDL2_LIBRARIES}
 	${SDL2_IMAGE_LIBRARIES}
 	${SIGCPP_LIBRARIES}