update README: build openssl env var

README.md

@@ -41,7 +41,8 @@ Exactly the goals I set out to achieve!
 - Q: **Does ZAP support TLS / HTTPS?**
 - A: Yes, ZAP supports using the system's openssl. See the
   [https](./examples/https/https.zig) example and make sure to build with
-  `-Dopenssl=true`.
+  the environment variable `ZAP_USE_OPENSSL=true`:
+  - `ZAP_USE_OPENSSL=true zig build https`
 
 ## Here's what works
 
@@ -116,8 +117,11 @@ necessary to show a feature.
   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