Be more explicit how to turn crypto support off.
diff --git a/INSTALL.md b/INSTALL.md
index c440990..8f9e7b2 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -16,7 +16,8 @@
 - [GnuTLS](https://www.gnutls.org/).
 
 If you don't want a library even if it is installed, you can
-pass `-DENABLE_<LIBRARY>=OFF` to cmake.
+pass `-DENABLE_<LIBRARY>=OFF` to cmake, where `LIBRARY` is one of
+`COMMONCRYPTO`, `OPENSSL`, or `GNUTLS`.
 
 The basic usage is
 ```sh