Clarify usage of OPT by noting that if it is set the default values will be
left out.
diff --git a/Misc/SpecialBuilds.txt b/Misc/SpecialBuilds.txt
index 39dc1cb..feff20e 100644
--- a/Misc/SpecialBuilds.txt
+++ b/Misc/SpecialBuilds.txt
@@ -2,7 +2,8 @@
compile-time preprocessor defines.
It is best to define these options in the OPT environment variable;
-``OPT="-DPy_REF_DEBUG" ./configure``.
+``OPT="-DPy_REF_DEBUG" ./configure``. If you want the default values of
+OPT to also be included you will need to add them in yourself manually.
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Py_REF_DEBUG introduced in 1.4