update README: build openssl env var

README.md

@@ -34,7 +34,9 @@ Exactly the goals I set out to achieve!
   on Windows are WSL2 or a docker container.
 - Q: **Does ZAP support TLS / HTTPS?**
 - A: Yes, ZAP supports using the system's openssl. See the
-  [https](./examples/https/https.zig) example.
+  [https](./examples/https/https.zig) example and make sure to build with
+  the environment variable `ZAP_USE_OPENSSL=true`:
+  - `ZAP_USE_OPENSSL=true zig build https`
 - Q: **Are there API docs?**
 - A: They are under development. Git clone this, then run `zig build run-docserver`.
 
@@ -115,8 +117,11 @@ port and docs dir: `zig build docserver && zig-out/bin/docserver --port=8989
   call `r.sendError(err, status_code)` when you catch an error and a stack trace
   will be returned to the client / browser.
 - [**HTTPS**](examples/https/https.zig): Shows how easy it is to use facil.io's
-  openssl support. Must be compiled with the `-Dopenssl=true` and requires
-  openssl dev dependencies (headers, lib) to be installed on the system.
+  openssl support. Must be compiled with the environment variable
+  `ZAP_USE_OPENSSL` set to `true` and requires openssl dev dependencies
+  (headers, lib) to be installed on the system.
+  - run it like this: `ZAP_USE_OPENSSL=true zig build run-https`
+  - it will tell you how to generate certificates
 
 I'll continue wrapping more of facil.io's functionality and adding stuff to zap
 to a point where I can use it as the JSON REST API backend for real research