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/Makefile.am

@@ -594,7 +594,7 @@ libzcash_a_CPPFLAGS = -DMULTICORE -fopenmp -fPIC -DBOOST_SPIRIT_THREADSAFE -DHAV
 #libzcash_a_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS) $(LIBTOOL_APP_LDFLAGS)
 #libzcash_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) -DMONTGOMERY_OUTPUT
 
-libzcash_a_CXXFLAGS = $(SAN_CXXFLAGS) $(HARDENED_CXXFLAGS) -fwrapv -fno-strict-aliasing -std=gnu17
+libzcash_a_CXXFLAGS = $(SAN_CXXFLAGS) $(HARDENED_CXXFLAGS) -fwrapv -fno-strict-aliasing -std=gnu++17
 libzcash_a_LDFLAGS = $(SAN_LDFLAGS) $(HARDENED_LDFLAGS)
 libzcash_a_CPPFLAGS += -DMONTGOMERY_OUTPUT
 
@@ -636,7 +636,7 @@ libhush_a_SOURCES = \
 
 libhush_a_CPPFLAGS = -DMULTICORE -fopenmp -fPIC -DBINARY_OUTPUT -DCURVE_ALT_BN128 -DBOOST_SPIRIT_THREADSAFE -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS $(HARDENED_CPPFLAGS) -pipe -O1 -g  -Wstack-protector -fstack-protector-all -fPIE -fvisibility=hidden -DSTATIC $(BITCOIN_INCLUDES)
 
-libhush_a_CXXFLAGS = $(HARDENED_CXXFLAGS) -fwrapv -fno-strict-aliasing -std=gnu17
+libhush_a_CXXFLAGS = $(HARDENED_CXXFLAGS) -fwrapv -fno-strict-aliasing -std=gnu++17
 
 libhush_a_LDFLAGS = $(HARDENED_LDFLAGS)