DOC: add a few words about USE_* and the addons directory
Nowhere it was really explained what USE_* was used for. Let's
take this opportunity to introduce addons/ which will also rely
on these.
diff --git a/INSTALL b/INSTALL
index 32c0dd3..c07a68c 100644
--- a/INSTALL
+++ b/INSTALL
@@ -71,14 +71,25 @@
Most configuration variables are in fact booleans. Some options are detected and
enabled by default if available on the target platform. This is the case for all
those named "USE_<feature>". These booleans are enabled by "USE_<feature>=1"
-and are disabled by "USE_<feature>=" (with no value). The last occurrence on the
-command line overrides any previous one. Example :
+and are disabled by "USE_<feature>=" (with no value). An exhaustive list of the
+supported USE_* features is located at the top of the main Makefile. The last
+occurrence of such an option on the command line overrides any previous one.
+Example :
$ make TARGET=generic USE_THREAD=
In case of error or missing TARGET, a help screen is displayed. It is also
possible to display a list of all known options using "make help".
+Some optional components which may depend on third-party libraries, are used
+with popular tools which are not necessarily standard implementations, or are
+maintained at slower pace than the core of the project, are located in the
+"addons/" directory. These ones may disappear in a future version if the
+product they depend on disappears or if their maintainers do not assign enough
+resources to maintain them any more. For this reason they are not built by
+default, but some USE_* options are usually provided for them, and their build
+is routinely tested anyway.
+
3) Build environment
====================