Add build script to STM

It will automatically detect WASM compilation and will deactivate batch verification
of aggregates, provided by BLST, which is not available in WASM.

mithril-stm/build.rs

@@ -0,0 +1,11 @@
+use std::env;
+
+fn main() {
+    let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
+    let target_no_batch_verify_aggregates =
+        target_arch.eq("wasm32") || env::var("STM_TEST_NO_BATCH_VERIFY_AGGREGATES").is_ok();
+
+    if !target_no_batch_verify_aggregates {
+        println!("cargo:rustc-cfg=feature=\"batch-verify-aggregates\"");
+    }
+}

mithril-stm/src/multi_sig.rs

@@ -404,6 +404,7 @@ impl Signature {
     }
 
     /// Batch verify several sets of signatures with their corresponding verification keys.
+    #[cfg(feature = "batch-verify-aggregates")]
     pub fn batch_verify_aggregates(
         msgs: &[Vec<u8>],
         vks: &[VerificationKey],

mithril-stm/src/stm.rs

@@ -789,6 +789,7 @@ impl<D: Clone + Digest + FixedOutput + Send + Sync> StmAggrSig<D> {
     }
 
     /// Batch verify a set of signatures, with different messages and avks.
+    #[cfg(feature = "batch-verify-aggregates")]
     pub fn batch_verify(
         stm_signatures: &[Self],
         msgs: &[Vec<u8>],