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

depends/builders/darwin.mk

@@ -1,5 +1,5 @@
-build_darwin_CC = gcc-8
-build_darwin_CXX = g++-8
+build_darwin_CC = gcc-15
+build_darwin_CXX = g++-15
 build_darwin_AR: = $(shell xcrun -f ar)
 build_darwin_RANLIB: = $(shell xcrun -f ranlib)
 build_darwin_STRIP: = $(shell xcrun -f strip)
@@ -10,8 +10,8 @@ build_darwin_SHA256SUM = shasum -a 256
 build_darwin_DOWNLOAD = curl --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -L -f -o
 
 #darwin host on darwin builder. overrides darwin host preferences.
-darwin_CC= gcc-8
-darwin_CXX= g++-8
+darwin_CC= gcc-15
+darwin_CXX= g++-15
 darwin_AR:=$(shell xcrun -f ar)
 darwin_RANLIB:=$(shell xcrun -f ranlib)
 darwin_STRIP:=$(shell xcrun -f strip)