cmake/gsdx: check png++ is installed to support png image

Otherwise fallback to bmp file

cmake/SearchForStuff.cmake

@@ -72,6 +72,7 @@ endif()
 check_lib(EGL EGL EGL/egl.h)
 check_lib(PORTAUDIO portaudio portaudio.h pa_linux_alsa.h)
 check_lib(SOUNDTOUCH SoundTouch soundtouch/SoundTouch.h)
+check_lib(PNGPP FALSE png++/png.hpp)
 
 if(SDL2_API)
     check_lib(SDL2 SDL2 SDL.h PATH_SUFFIXES SDL2)

plugins/GSdx/CMakeLists.txt

@@ -38,6 +38,10 @@ if(XDG_STD)
     set(GSdxFinalFlags ${GSdxFinalFlags} -DXDG_STD)
 endif()
 
+if(PNGPP_FOUND)
+    set(GSdxFinalFlags ${GSdxFinalFlags} -DPNGPP_SUPPORTED)
+endif()
+
 #Clang doesn't support a few common flags that GCC does.
 if(NOT USE_CLANG)
     set(GSdxFinalFlags ${GSdxFinalFlags} -fabi-version=6)

plugins/GSdx/config.h

@@ -49,7 +49,7 @@
 //#define ENABLE_OPENCL
 #endif
 
-#if defined(ENABLE_OGL_DEBUG) && defined(__linux__)
+#if defined(ENABLE_OGL_DEBUG) && defined(__linux__) && defined(PNGPP_SUPPORTED)
 // Allow to dump texture as PNG (require libpng++). It reduces the size of the dump
 // and alpha is well supported (on linux)
 #define ENABLE_OGL_PNG