Compiler extensions are disabled before the first `project` call to
force CMake into considering compilers to not support GNU extensions
(even though clang and gcc do).
Also sets common clang compiler options so they can be used across all
supported platforms.
`openmp-simd` support is enabled by default as there is no performance
penalty on x86_64 systems and intrinsics are enabled on arm64.
Also implements CMake's `CMAKE_COMPILE_WARNING_AS_ERROR` flag to
enable the desired behavior and configuration time.
PatTheMav