Add a line explaining what env var to use to set the compiler flags and an
example of how to do it so that it is passed directly to ./configure .
diff --git a/Misc/SpecialBuilds.txt b/Misc/SpecialBuilds.txt
index 1919e66..39dc1cb 100644
--- a/Misc/SpecialBuilds.txt
+++ b/Misc/SpecialBuilds.txt
@@ -1,6 +1,9 @@
This file describes some special Python build types enabled via
compile-time preprocessor defines.
+It is best to define these options in the OPT environment variable;
+``OPT="-DPy_REF_DEBUG" ./configure``.
+
---------------------------------------------------------------------------
Py_REF_DEBUG introduced in 1.4
named REF_DEBUG before 1.4