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

util/build-mac.sh

@@ -3,8 +3,8 @@
 # Distributed under the GPLv3 software license, see the accompanying
 # file COPYING or https://www.gnu.org/licenses/gpl-3.0.en.html
 
-export CC=gcc-8
-export CXX=g++-8
+export CC=gcc-15
+export CXX=g++-15
 export LIBTOOL=libtool
 export AR=ar
 export RANLIB=ranlib
@@ -34,16 +34,24 @@ fi
 # If --enable-lcov is the first argument, enable lcov coverage support:
 LCOV_ARG=''
 HARDENING_ARG='--disable-hardening'
+TEST_ARG=''
 if [ "x${1:-}" = 'x--enable-lcov' ]
 then
     LCOV_ARG='--enable-lcov'
     HARDENING_ARG='--disable-hardening'
     shift
+elif [ "x${1:-}" = 'x--disable-tests' ]
+then
+    TEST_ARG='--enable-tests=no'
+    shift
 fi
 
 TRIPLET=`./depends/config.guess`
 PREFIX="$(pwd)/depends/$TRIPLET"
 
+# Ensure system Rust is in PATH for modern macOS compatibility
+export PATH="$HOME/.cargo/bin:$PATH"
+
 make "$@" -C ./depends/ V=1 NO_QT=1
 
 #BUILD CCLIB
@@ -68,7 +76,7 @@ cd $WD
 
 ./autogen.sh
 CPPFLAGS="-I$PREFIX/include -arch x86_64" LDFLAGS="-L$PREFIX/lib -arch x86_64 -Wl,-no_pie" \
-CXXFLAGS='-arch x86_64 -I/usr/local/Cellar/gcc\@8/8.3.0/include/c++/8.3.0/ -I$PREFIX/include -fwrapv -fno-strict-aliasing -Wno-builtin-declaration-mismatch -Werror -g -Wl,-undefined -Wl,dynamic_lookup' \
-./configure --prefix="${PREFIX}" --with-gui=no "$HARDENING_ARG" "$LCOV_ARG"
+CXXFLAGS='-arch x86_64 -I/usr/local/Cellar/gcc/15.2.0_1/include/c++/15/ -I$PREFIX/include -fwrapv -fno-strict-aliasing -Wno-builtin-declaration-mismatch -Werror -Wno-error=deprecated-declarations -g -Wl,-undefined -Wl,dynamic_lookup' \
+./configure --prefix="${PREFIX}" --with-gui=no "$HARDENING_ARG" "$LCOV_ARG" $TEST_ARG
 
 make "$@" V=1 NO_GTEST=1 STATIC=1