CMake: split out atsc-encoder into its own CMakeLists

Also remove dependency of third_party/astc-encoder on
src/System/Debug.hpp.

Bug: b/145758253
Change-Id: I329928d9221c99a416fcdb1b0dd44d2578a22cb8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43269
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
diff --git a/src/Device/ASTC_Decoder.cpp b/src/Device/ASTC_Decoder.cpp
index aef8795..7a289ad 100644
--- a/src/Device/ASTC_Decoder.cpp
+++ b/src/Device/ASTC_Decoder.cpp
@@ -17,7 +17,7 @@
 #include "System/Math.hpp"
 
 #ifdef SWIFTSHADER_ENABLE_ASTC
-#	include "../third_party/astc-encoder/Source/astc_codec_internals.h"
+#	include "astc_codec_internals.h"
 #endif
 
 #include <memory>