build.rs: Remove DEBUG environment variable

The DEBUG variable is only removed it set to `false` by cargo. When
called with DEBUG set, AFLplusplus will set -Werror in CFLAGS and
sometimes fail to build spuriously.

build.rs

@@ -28,6 +28,9 @@ fn build_afl(out_dir: &Path) {
         .env("NO_BUILD", "1")
         .env("DESTDIR", out_dir)
         .env("PREFIX", "");
+    if let Ok("false") = std::env::var("DEBUG").as_deref() {
+        command.env_remove("DEBUG");
+    }
     let status = command.status().expect("could not run 'make'");
     assert!(status.success());
 }