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/wallet/wallet.cpp

@@ -39,6 +39,8 @@
 #include "coins.h"
 #include "wallet/asyncrpcoperation_saplingconsolidation.h"
 #include "wallet/asyncrpcoperation_sweep.h"
+#include <random>
+#include <limits>
 #include "zcash/zip32.h"
 #include "cc/CCinclude.h"
 #include <assert.h>
@@ -3416,7 +3418,7 @@ bool CWallet::SelectCoinsMinConf(const CAmount& nTargetValue, int nConfMine, int
     vector<pair<CAmount, pair<const CWalletTx*,unsigned int> > > vValue;
     CAmount nTotalLower = 0;
 
-    random_shuffle(vCoins.begin(), vCoins.end(), GetRandInt);
+    std::shuffle(vCoins.begin(), vCoins.end(), std::mt19937(GetRand(std::numeric_limits<uint32_t>::max())));
 
     BOOST_FOREACH(const COutput &output, vCoins)
     {