Add util function for boolean env vars

It's probably good to be consistent about what is recognized as "true"
without copying the same code.

src/utils/env.rs

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
+pub fn bool_var(name: &str) -> Option<bool> {
+    let value = std::env::var(name).ok()?.to_lowercase();
+    Some(["1", "true", "yes", "y"].contains(&value.as_str()))
+}

src/utils/mod.rs

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
+pub mod env;
 mod ids;
 pub(crate) use self::ids::id_gen;
 pub mod geometry;