Fix macOS Sequoia build with GCC 15 and update README

- Update compiler references from gcc-8 to gcc-15 across build system
  (build-mac.sh, darwin.mk, Makefile_custom)
- Use system Rust (rustup) instead of bundled Rust 1.32.0 for librustzcash
  to fix rlib linker incompatibility on macOS Sequoia
- Replace deprecated std::random_shuffle with std::shuffle (net.cpp,
  transaction_builder.cpp, wallet.cpp)
- Fix -std=gnu17 -> -std=gnu++17 for C++ targets (libzcash, libhush)
- Fix nodiscard warning in glibcxx_sanity.cpp
- Replace deprecated OSMemoryBarrier with std::atomic_thread_fence in LevelDB
- Add -Wno-error=deprecated-declarations to CXXFLAGS for third-party headers
- Fix REMAINING_ARGS unbound variable in build.sh
- Add --disable-tests handling to build-mac.sh
- Update README with correct macOS build dependencies and instructions

build.sh

@@ -175,21 +175,21 @@ if [ $BUILD_LINUX_COMPAT -eq 1 ] || [ $BUILD_LINUX_RELEASE -eq 1 ] || [ $BUILD_W
     if [ $BUILD_LINUX_RELEASE -eq 1 ]; then
         echo "=== Building Linux release ==="
         clean_for_platform linux
-        ./util/build.sh --disable-tests "${REMAINING_ARGS[@]}"
+        ./util/build.sh --disable-tests ${REMAINING_ARGS[@]+"${REMAINING_ARGS[@]}"}
         package_release linux-amd64
     fi
     
     if [ $BUILD_WIN_RELEASE -eq 1 ]; then
         echo "=== Building Windows release ==="
         clean_for_platform windows
-        ./util/build-win.sh --disable-tests "${REMAINING_ARGS[@]}"
+        ./util/build-win.sh --disable-tests ${REMAINING_ARGS[@]+"${REMAINING_ARGS[@]}"}
         package_release win64
     fi
     
     if [ $BUILD_MAC_RELEASE -eq 1 ]; then
         echo "=== Building macOS release ==="
         clean_for_platform macos
-        ./util/build-mac.sh --disable-tests "${REMAINING_ARGS[@]}"
+        ./util/build-mac.sh --disable-tests ${REMAINING_ARGS[@]+"${REMAINING_ARGS[@]}"}
         package_release macos
     fi
     
@@ -201,11 +201,11 @@ fi
 
 # Standard build (auto-detect OS)
 if [[ "$OSTYPE" == "linux-gnu"* ]]; then
-    ./util/build.sh --disable-tests "${REMAINING_ARGS[@]}"
+    ./util/build.sh --disable-tests ${REMAINING_ARGS[@]+"${REMAINING_ARGS[@]}"}
 elif [[ "$OSTYPE" == "darwin"* ]]; then
-    ./util/build-mac.sh --disable-tests "${REMAINING_ARGS[@]}"
+    ./util/build-mac.sh --disable-tests ${REMAINING_ARGS[@]+"${REMAINING_ARGS[@]}"}
 elif [[ "$OSTYPE" == "msys"* ]]; then
-    ./util/build-win.sh --disable-tests "${REMAINING_ARGS[@]}"
+    ./util/build-win.sh --disable-tests ${REMAINING_ARGS[@]+"${REMAINING_ARGS[@]}"}
 else
     echo "Unable to detect your OS. What are you using?"
 fi