Add further LLVM usage notes to the release notes

* documentation/release-notes/source/2019.1.rst: Explain more about
  requirements for using the LLVM back-end.

documentation/release-notes/source/2019.1.rst

@@ -29,6 +29,19 @@ Note that LLVM is not yet the default back end so for now you must add
 ``-back-end llvm`` to the command line when building if you want LLVM.  We
 expect to make it the default in an upcoming release.
 
+Note also that the binary releases do not assume that you have a
+sufficiently new release of the LLVM Clang compiler, and are
+configured to use the platform default C compiler instead. If you have
+installed Clang 7.0 or later, you can edit
+``share/opendylan/build-scripts/config.jam`` and set the CC variable
+to point to your Clang executable:
+
+.. code-block:: jam
+   CC                ?= clang-7 ;
+   CCFLAGS           ?= -w -g -O2 ;
+   C++               ?= clang++-7 ;
+   C++FLAGS          ?= $(CCFLAGS) ;
+
 Compiler
 ========