DSP_INIT can be called from multiple threads (#46)

Author: deven96

build.rs

@@ -93,6 +93,7 @@ fn setup_build(build: &mut cc::Build, include_dir: &PathBuf) {
     build.include(include_dir);
     build.define("NDEBUG", Some("1"));
     build.define("_THREAD_SAFE", Some("1"));
+    build.define("WEBP_USE_THREAD", Some("1"));
     if !build.get_compiler().is_like_msvc() {
         build.flag("-fvisibility=hidden").flag("-Wall");
     } else {