[changelog] Add changes in dunecontrol's configure-opts

CHANGELOG.md

@@ -35,3 +35,12 @@
   [Vc](https://github.com/VcDevel/Vc) as field types.
 
     See core/dune-common!121
+
+## build-system
+
+- Variables passed via `dunecontrol`'s command `--configure-opts=..` are now
+  added to the CMake flags.
+
+- Bash-style variables which are passed to `dunecontrol`'s command `configure-opts`
+  are no longer transformed to their equivalent CMake command. Pass
+  `-DCMAKE_C_COMPILER=gcc` instead of `CC=gcc`.