Document OPENSSL_PREFIX

Author: eregon

doc/user/installing-libssl.md

@@ -12,6 +12,11 @@ TruffleRuby supports both versions 1.0.2 and 1.1.0.
 If you experience `openssl`-related errors, it might help to recompile the `openssl` module by running `lib/truffle/post_install_hook.sh`.
 This is done automatically by Ruby managers, and mentioned in the post-install message when installing TruffleRuby via `gu install` in GraalVM.
 
+To compile TruffleRuby against a non-system `libssl`, set `OPENSSL_PREFIX` while installing TruffleRuby:
+```bash
+export OPENSSL_PREFIX=/path/to/my/openssl-1.1.0
+```
+
 ### RedHat-based: Fedora, Oracle Linux, etc
 
 ```bash