build.rs: patch out any calls to printf

Author: apoelstra

secp256k1-sys/build.rs

@@ -20,11 +20,16 @@ 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
+               .flag_if_supported("-Wno-unused-parameter") // patching out printf causes this warning
                .define("SECP256K1_API", Some(""))
                .define("ENABLE_MODULE_ECDH", Some("1"))
                .define("ENABLE_MODULE_SCHNORRSIG", Some("1"))
                .define("ENABLE_MODULE_EXTRAKEYS", Some("1"))
-               .define("ENABLE_MODULE_ELLSWIFT", Some("1"));
+               .define("ENABLE_MODULE_ELLSWIFT", Some("1"))
+               // upstream sometimes introduces calls to printf, which we cannot compile
+               // with WASM due to its lack of libc. printf is never necessary and we can
+               // just #define it away.
+               .define("printf(...)", Some(""));
 
     if cfg!(feature = "lowmemory") {
         base_config.define("ECMULT_WINDOW_SIZE", Some("4")); // A low-enough value to consume negligible memory