Fix: ***WARNING*** Library was built as DEBUG. Timings may be affected.

[MarkReedZ]

When google benchmark is built with cmake you must define NDEBUG when doing a release build or it will build in asserts and complain with

***WARNING*** Library was built as DEBUG. Timings may be affected.

[MarkReedZ]

Do we need to add a CI to test against various platforms?

[ashvardanian]

No need for CI, but would be great to use target-specific definitions.

[MarkReedZ]

Done.

  # Remove the google benchmark built in debug warning
  if (CMAKE_BUILD_TYPE STREQUAL "Release")
    target_compile_definitions( benchmark PRIVATE NDEBUG ) 
  endif()