Fix warnings

1) SECP256k1_BUILD is already set, so setting to Some("") avoids the
duplicate warning
2) Misc rust unused warnings in various feature flag combinations

secp256k1-zkp-sys/build.rs

@@ -38,7 +38,7 @@ fn main() {
         .include("depend/secp256k1/include")
         .include("depend/secp256k1/src")
         .flag_if_supported("-Wno-unused-function") // some ecmult stuff is defined but not used upstream
-        .define("SECP256K1_BUILD", Some("1"))
+        .define("SECP256K1_BUILD", Some(""))
         .define("ENABLE_MODULE_SURJECTIONPROOF", Some("1"))
         .define("ENABLE_MODULE_GENERATOR", Some("1"))
         .define("ENABLE_MODULE_RANGEPROOF", Some("1"))

src/zkp/ecdsa_adaptor.rs

@@ -287,9 +287,11 @@ impl EcdsaAdaptorSignature {
 mod tests {
     use super::Message;
     use super::*;
-    use rand::{rngs::ThreadRng, thread_rng, RngCore};
     use SECP256K1;
+    #[cfg(not(rust_secp_fuzz))]
+    use rand::{rngs::ThreadRng, thread_rng, RngCore};
 
+    #[cfg(not(rust_secp_fuzz))]
     fn test_ecdsa_adaptor_signature_helper(
         encrypt: fn(&Message, &SecretKey, &PublicKey, &mut ThreadRng) -> EcdsaAdaptorSignature,
     ) {

src/zkp/generator.rs

@@ -177,6 +177,7 @@ impl Generator {
     ///
     /// This is `pub(crate)` because generators have a different serialization from regular public keys.
     /// As such, certain invariants need to be upheld which is easier if we don't allow users to access the internal representation of generators.
+    #[cfg(feature = "std")] // for un-used warnings
     pub(crate) fn as_inner(&self) -> &ffi::PublicKey {
         &self.0
     }

src/zkp/tag.rs

@@ -12,7 +12,7 @@ impl Tag {
         self.0
     }
 
-    #[cfg(feature = "rand")]
+    #[cfg(all(feature = "rand", feature = "std"))]
     pub(crate) fn as_inner(&self) -> &ffi::Tag {
         &self.0
     }