Change libav* autodetection to support framedumping on Ubuntu 14.04

Add an "ugly" workaround in the AVIDump code, but looking at other project this
seems to be the most common way to handle this API change.

CMakeTests/CheckLib.cmake

@@ -55,8 +55,8 @@ endmacro()
 
 macro(check_libav)
 	if(PKG_CONFIG_FOUND)
-		pkg_check_modules(LIBAV libavcodec>=55.52.102 libavformat>=55.33.100
-			libswscale>=2.5.102 libavutil>=52.66.100)
+		pkg_check_modules(LIBAV libavcodec>=54.35.0 libavformat>=54.20.4
+			libswscale>=2.1.1 libavutil>=52.3.0)
 	else()
 		message("pkg-config is required to check for libav/ffmpeg")
 	endif()

Source/Core/VideoCommon/AVIDump.cpp

@@ -215,6 +215,11 @@ extern "C" {
 #include <libavutil/mathematics.h>
 }
 
+#if LIBAVCODEC_VERSION_INT < AV_VERSION_INT(55, 28, 1)
+#define av_frame_alloc avcodec_alloc_frame
+#define av_frame_free avcodec_free_frame
+#endif
+
 static AVFormatContext* s_format_context = nullptr;
 static AVStream* s_stream = nullptr;
 static AVFrame* s_src_frame = nullptr;