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/transaction_builder.cpp

@@ -11,6 +11,8 @@
 #include <boost/optional/optional_io.hpp>
 #include <librustzcash.h>
 #include "zcash/Note.hpp"
+#include <random>
+#include <limits>
 extern bool fZDebug;
 
 SpendDescriptionInfo::SpendDescriptionInfo(
@@ -66,7 +68,7 @@ void TransactionBuilder::AddSaplingOutput(
 void TransactionBuilder::ShuffleOutputs()
 {
     LogPrintf("%s: Shuffling %d zouts\n", __func__, outputs.size() );
-    random_shuffle( outputs.begin(), outputs.end(), GetRandInt );
+    std::shuffle( outputs.begin(), outputs.end(), std::mt19937(GetRand(std::numeric_limits<uint32_t>::max())) );
 }
 
 void TransactionBuilder::AddTransparentInput(COutPoint utxo, CScript scriptPubKey, CAmount value, uint32_t _nSequence)