Use BoringSSL 5.x's FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION

BoringSSL 5.x consolidated the older
`BORINGSSL_UNSAFE_DETERMINISTIC_MODE` and
`BORINGSSL_UNSAFE_FUZZER_MODE` preprocessor switches into a single
`FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION` flag. The two old defines
no longer exist anywhere in the new BoringSSL tree.

With our build.rs still passing only the old defines when
`quiche/fuzzing` was enabled:

  - The deterministic RNG implementation
    (`crypto/rand/deterministic.cc`) is now guarded by
    `OPENSSL_RAND_DETERMINISTIC`, which is in turn defined only when
    `FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION` is set. So that file
    compiled to nothing and `RAND_reset_for_fuzzing` was never
    emitted.

  - `quiche-fuzz/src/lib.rs` declares the function as `extern "C"`
    and calls it, so `cargo fuzz build` failed at link time with
    `undefined symbol: RAND_reset_for_fuzzing`.

Replace the two old defines with the new single one. Verified
locally that `cargo build -p quiche --features=fuzzing` succeeds and
the resulting `libcrypto.a` exports `RAND_reset_for_fuzzing`.

Author: cjpatton

quiche/src/build.rs

@@ -383,10 +383,15 @@ fn main() {
             let mut cfg = get_boringssl_cmake_config(&bssl_src);
 
             if cfg!(feature = "fuzzing") {
-                cfg.cxxflag("-DBORINGSSL_UNSAFE_DETERMINISTIC_MODE")
-                    .cxxflag("-DBORINGSSL_UNSAFE_FUZZER_MODE");
-                cfg.cflag("-DBORINGSSL_UNSAFE_DETERMINISTIC_MODE")
-                    .cflag("-DBORINGSSL_UNSAFE_FUZZER_MODE");
+                // BoringSSL 5.x consolidated the older
+                // `BORINGSSL_UNSAFE_{DETERMINISTIC,FUZZER}_MODE`
+                // defines into the single
+                // `FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION` switch.
+                // Setting it makes BoringSSL build the deterministic
+                // RNG path (so `RAND_reset_for_fuzzing` is exported)
+                // and enables the fuzzer-mode TLS shortcuts.
+                cfg.cflag("-DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION")
+                    .cxxflag("-DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION");
             }
 
             cfg.build_target("ssl").build();