Update changelog

* Detail the `GNUInstallDirs` changes
* Non-Windows builds now use the absolute path for all
  data paths. This is less brittle, as relative path
  lookup in Unix always depends on the current value of
  the PWD environmental variable.

changelog

@@ -4,11 +4,18 @@ Version 1.13.10+dev:
      have linger=no in [end_level].
    * A Tale of Two Brothers:
      * Added a full set of hints on easy difficulty.
- * Contiuous Integration (CI) and Unit testing
+ * Packaging
    * Renamed the target and binary for the Boost unit tests from `test` to
      `boost_unit_tests`. This quells warnings from CMake about reserved target
      names, and reduces confusion about which `test` is intended: the wesnoth
      executable or the standard shell command.
+   * The CMake build system now uses standard Kitware-supported variables
+     from the `GNUInstallDirs` module. Distributions might have to change
+     the defaults to suit their FHS needs.
+   * The previous addition of `GNUInstallDirs` necessitates a bumping of the
+     CMake minimum requirements, which is now at 2.8.5. Most distributions
+     already use a CMake version above 3.0, so this would only affect users
+     of vintage CMake versions.
  * Language and i18n:
    * Updated translations: Chinese (Simplified), Italian
  * Lua API: