RELNOTES: add build items

Relates to #5859 #5864 #5866.

Author: kmk3

RELNOTES

@@ -26,6 +26,9 @@ firejail (0.9.73) baseline; urgency=low
   * build: deb: enable apparmor by default & remove deb-apparmor (#5668)
   * build: Fix whitespace and add .editorconfig (#5674)
   * build: enable compiler warnings by default (#5842)
+  * build: remove -mretpoline and NO_EXTRA_CFLAGS (#5859)
+  * build: disable all built-in implicit make rules (#5864)
+  * build: organize and standardize make vars and targets (#5866)
   * ci: always update the package db before installing packages (#5742)
   * ci: fix codeql unable to download its own bundle (#5783)
   * ci: split configure/build/install commands on gitlab (#5784)