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

depends/strip-rlib-metadata.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+# Strip rust.metadata.bin from .rlib archives for macOS linker compatibility
+RLIB_DIR="$1"
+if [ -d "$RLIB_DIR" ]; then
+    for rlib in "$RLIB_DIR"/*.rlib; do
+        [ -f "$rlib" ] && ar d "$rlib" rust.metadata.bin 2>/dev/null || true
+    done
+fi