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/packages/librustzcash.mk

@@ -41,7 +41,7 @@ define $(package)_preprocess_cmds
 endef
 
 define $(package)_build_cmds
-  $(host_prefix)/native/bin/cargo build --package librustzcash $($(package)_build_opts)
+  CARGO=$(HOME)/.cargo/bin/cargo RUSTC=$(HOME)/.cargo/bin/rustc $(HOME)/.cargo/bin/cargo build --package librustzcash $($(package)_build_opts)
 endef
 
 define $(package)_stage_cmds