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

README.md

@@ -106,19 +106,32 @@ apt-get install -y gcc-7 g++-7 && \
 
 # Build on Mac
 
-```
-sudo port update
-sudo port upgrade outdated
-sudo port install qt5
+Install Xcode Command Line Tools and [Homebrew](https://brew.sh/), then install dependencies:
+
+```sh
+xcode-select --install
+brew install gcc autoconf automake pkgconf libtool cmake curl
+
+# Install Rust (needed for librustzcash on macOS Sequoia+)
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+source "$HOME/.cargo/env"
 
 # clone git repo
 git clone https://git.hush.is/hush/hush3
 cd hush3
-# Build
-# This uses 3 build processes, you need 2GB of RAM for each. 
+# Build (uses 3 build processes, you need 2GB of RAM for each)
+# Make sure libtool gnubin and cargo are on PATH
+export PATH="$HOME/.cargo/bin:/usr/local/opt/libtool/libexec/gnubin:$PATH"
 ./build.sh -j3
 ```
 
+For a release build:
+
+```sh
+export PATH="$HOME/.cargo/bin:/usr/local/opt/libtool/libexec/gnubin:$PATH"
+./build.sh --mac-release -j$(sysctl -n hw.ncpu)
+```
+
 # Installing Hush binaries
 
 1. [Download the release](https://git.hush.is/hush/hush3/releases) with a .deb file extension.