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/net.cpp

@@ -33,6 +33,8 @@
 #include "crypto/common.h"
 #include "hush/utiltls.h"
 #include <random.h>
+#include <random>
+#include <limits>
 #ifdef _WIN32
 #include <string.h>
 #else
@@ -2004,7 +2006,7 @@ void ThreadMessageHandler()
         // Randomize the order in which we process messages from/to our peers.
         // This prevents attacks in which an attacker exploits having multiple
         // consecutive connections in the vNodes list.
-        random_shuffle(vNodesCopy.begin(), vNodesCopy.end(), GetRandInt);
+        std::shuffle(vNodesCopy.begin(), vNodesCopy.end(), std::mt19937(GetRand(std::numeric_limits<uint32_t>::max())));
 
         BOOST_FOREACH(CNode* pnode, vNodesCopy)
         {
@@ -2516,7 +2518,7 @@ void RelayTransaction(const CTransaction& tx, const CDataStream& ss)
     // We always round down, except when we have only 1 connection
     auto newSize = (vNodes.size() / 2) == 0 ? 1 : (vNodes.size() / 2);
 
-    random_shuffle( vRelayNodes.begin(), vRelayNodes.end(), GetRandInt );
+    std::shuffle( vRelayNodes.begin(), vRelayNodes.end(), std::mt19937(GetRand(std::numeric_limits<uint32_t>::max())) );
 
     vRelayNodes.resize(newSize);
     if (HUSH_TESTNODE==1 && vNodes.size() == 0) {