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

src/cc/Makefile_custom

@@ -1,5 +1,5 @@
 SHELL		= /bin/sh
-CC_DARWIN	= g++-8
+CC_DARWIN	= g++-15
 CC_WIN		= x86_64-w64-mingw32-gcc-posix
 CC_AARCH64	= aarch64-linux-gnu-g++
 CFLAGS_DARWIN	= -DBUILD_CUSTOMCC -std=c++11 -arch x86_64 -I../secp256k1/include -I../../depends/$(shell echo `../..//depends/config.guess`/include) -I../univalue/include -I../leveldb/include -I.. -I. -fPIC -Wl,-undefined -Wl,dynamic_lookup -Wno-write-strings -shared -dynamiclib