Fix GUI-less build. Also potentially OS X. We now require libpng due …

…to our screenshot code relying on it now. WXWidgets links in libpng by itself so it isn't noticed in a wxwidgets build. OS X seems to not have libpng linked in from wxWidgets so just link in libpng at all times.

CMakeLists.txt

@@ -524,11 +524,16 @@ else()
 	set(LZO lzo2)
 endif()
 
-if(ANDROID)
+if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin" AND NOT ANDROID)
+	check_lib(PNG png png.h QUIET)
+endif()
+if (PNG_FOUND)
+	message("Using shared libpng")
+else()
 	message("Using static libpng from Externals")
 	add_subdirectory(Externals/libpng)
 	include_directories(Externals/libpng)
-	set(PNG libpng)
+	set(PNG png)
 endif()
 
 if(OPENAL_FOUND)

Source/Core/VideoCommon/CMakeLists.txt

@@ -39,8 +39,7 @@ set(SRCS	Src/BPFunctions.cpp
 			Src/XFMemory.cpp
 			Src/XFStructs.cpp
 			Src/memcpy_amd.cpp)
-
-set(LIBS core)
+set(LIBS core png)
 
 if(NOT _M_GENERIC)
 	set(SRCS ${SRCS}	Src/x64TextureDecoder.cpp