Add note about -D_REENTRANT for HP-UX 10.20.
diff --git a/README b/README
index 892a09c..8b31ca1 100644
--- a/README
+++ b/README
@@ -211,7 +211,11 @@
(The optimizer bug reported at this place in previous releases
has been worked around by a minimal code change.)
-HP-UX: Please read the file Misc/HPUX-NOTES.
+HP-UX: Please read the file Misc/HPUX-NOTES for shared libraries.
+ When using threading, you may have to add -D_REENTRANT to the
+ OPT variable in the top-level Makefile; reported by Pat Knight
+ this seems to make a difference (at least for HP-UX 10.20)
+ even though config.h defines it.
Minix: When using ack, use "CC=cc AR=aal RANLIB=: ./configure"!