build.rs instead of frontend crate features

frontend/build.rs

@@ -0,0 +1,22 @@
+use std::env;
+
+// https://doc.rust-lang.org/cargo/reference/build-scripts.html
+
+macro_rules! instruction {
+    ($($arg: tt)*) => {
+        println!($($arg)*)
+    }
+}
+
+// https://github.com/rust-lang/cargo/issues/985
+// macro_rules! warning {
+//     ($($arg: tt)*) => {
+//         instruction!("cargo:warning={}", format!($($arg)*))
+//     }
+// }
+
+fn main() {
+    let default_platform = "BROWSER";
+    let platform = env::var("FASTWAVE_PLATFORM").unwrap_or_else(|_| default_platform.to_owned());
+    instruction!("cargo:rustc-cfg=FASTWAVE_PLATFORM=\"{platform}\"");
+}