abort on CMake install w/ clear error

This updates the librustls CMakeLists.txt to abort on install. It's only
intended to be used for the C examples, which we _don't_ want anyone to
install anywhere!

Use cargo capi for librustls, not cmake.

```
$ cmake --install build
-- Install configuration: "Release"
CMake Error at build/cmake_install.cmake:46 (message):
  librustls installation via CMake is not supported.  Use 'cargo capi
  install' instead.

  See:
  https://github.com/rustls/rustls-ffi?tab=readme-ov-file#build-rustls-ffi
```

librustls/CMakeLists.txt

@@ -14,6 +14,13 @@ cmake_minimum_required(VERSION 3.15)
 
 project(rustls-ffi)
 
+install(
+    CODE
+        "message(FATAL_ERROR
+    \"librustls installation via CMake is not supported. Use 'cargo capi install' instead.\n\"
+    \"See: https://github.com/rustls/rustls-ffi?tab=readme-ov-file#build-rustls-ffi\")"
+)
+
 # Use `cmake -LH $BUILD_DIR` to see all options/help.
 # Use `cmake --build $BUILD_DIR --target help` to see all targets.