Align compilation flags across build systems

[mingxwa]

Changes

• Set CMAKE_CXX_EXTENSIONS to OFF. It defaults to ON, so CMake was the only toolchain compiling in GNU extension mode, -std=gnu++20 against -std=c++26 under Meson and -std=c++20 under Bazel. No effect on MSVC, where CMake maps standard and extension to the same -std:c++20.
• Added AppleClang to the -Wno-c++2b-extensions generator expressions in CMakeLists.txt and cmake/msft_proxy4ModuleTargets.cmake. CXX_COMPILER_ID reports AppleClang rather than Clang, while Meson and Bazel both classify it as clang.
• Defined NDEBUG consistently by setting b_ndebug in Meson and --compilation_mode=opt in Bazel. Only CMake defined it before, leaving PRO4D_DEBUG and assert compiled in under the other two.