[Build] Convert build system to raw cmake and remove premake layer

third_party/ffmpeg-xenia/premake5.lua

@@ -1,42 +0,0 @@
-group("third_party")
-
---
--- Call this function in subproject scope to set the needed include
--- dirs and defines.
---
-function ffmpeg_common()
-  defines({
-    "HAVE_AV_CONFIG_H",
-    "_USE_MATH_DEFINES", -- For M_PI/etc
-  })
-
-  -- Paths are relative to the calling script (sub-library premake files),
-  -- which are one level deeper (e.g. libavcodec/premake5.lua).
-  includedirs({
-    "..",           -- ffmpeg-xenia/ (config.h, avconfig.h, ffversion.h)
-    "../../FFmpeg", -- FFmpeg source headers
-  })
-  filter({"platforms:Windows", "configurations:Debug or configurations:Checked"})
-    optimize("Size") -- dead code elimination is mandatory
-    removebuildoptions({
-      "/RTCsu",      -- '/O1' and '/RTCs' command-line options are incompatible
-    })
-  filter({"platforms:Windows", "configurations:Release"})
-    linktimeoptimization("Off")
-  filter("platforms:Windows")
-    includedirs({
-      "../../FFmpeg/compat/atomics/win32",
-    })
-    links({
-      "bcrypt",
-    })
-  filter("platforms:Linux")
-    includedirs({
-      "../../FFmpeg/compat/atomics/gcc",
-    })
-  filter({})
-end
-
-include("libavcodec/premake5.lua")
-include("libavformat/premake5.lua")
-include("libavutil/premake5.lua")