Add -flto=auto to changelog

Author: solidpixel

Docs/ChangeLog-5x.md

@@ -17,11 +17,15 @@ The 5.3.0 release is a minor maintenance release.
   * **Feature:** Reference C builds (`ASTCENC_ISA_NONE`) now support compiling
     for big-endian CPUs. Compile with `-DASTCENC_BIG_ENDIAN=ON` when compiling
     for a big-endian target; it is not auto-detected.
+  * **Improvement:** Builds using GCC now specify `-flto=auto` to allow
+    parallel link steps, and remove the log warnings about not setting a CPU
+    count parameter value.
   * **Bug fix:** Builds using MSVC `cl.exe` that do not specify an explicit
     ISA using the preprocessor configuration defines will now correctly
     default to the SSE2 backend on x86-64 and the NEON backend on Arm64. Previously they would have defaulted to the reference C implementation,
     which is around 3.25 times slower.
 
+
 <!-- ---------------------------------------------------------------------- -->
 ## 5.2.0