common: Allow to configure exhaustive types when environment variable is set

Allow to configure exhaustive types via the
`RUMA_UNSTABLE_EXHAUSTIVE_TYPES` environment variable in the same way as
it's done in other crates.

ruma-common contains at least `NewPushRule` which may be configured
as exhaustive. `ruma-client-api` uses the mentioned type and, when the
environment variable is set, it fails to compile the project.

crates/ruma-common/CHANGELOG.md

@@ -26,6 +26,7 @@ Bug fixes:
 
 - `MatrixVersion::V1_0` now also matches Identity Service API versions r0.2.0 to
   r0.3.0.
+- Allow configure exhaustive types via `RUMA_UNSTABLE_EXHAUSTIVE_TYPES` environment variable
 
 Improvements:
 

crates/ruma-common/build.rs

@@ -7,4 +7,11 @@ fn main() {
     }
 
     println!("cargo:rerun-if-env-changed=RUMA_IDENTIFIERS_STORAGE");
+
+    // Set the `ruma_unstable_exhaustive_types` configuration from an environment variable.
+    if env::var("RUMA_UNSTABLE_EXHAUSTIVE_TYPES").is_ok() {
+        println!("cargo:rustc-cfg=ruma_unstable_exhaustive_types");
+    }
+
+    println!("cargo:rerun-if-env-changed=RUMA_UNSTABLE_EXHAUSTIVE_TYPES");
 }