CMakeCompiler Options Silently change -O3 to -O2

[adishavit]

On certain platforms, OpenCVCompilerOptions.cmake changes optimization flag -O3 to -O2.

This might have been important 5 years ago. s is still required?
It was originally supposed to be just for 32-bit MINGW, but that check is gone.

Specifically, it prevents -O3 when building OpenCV with emscripten, which may significantly reduce the optimizations. It's possible to just check for NOT EMSCRIPTEN but it may be worth reconsidering this whole silent change as it may not be relevant anymore.

[adishavit]

This is especially strange since OpenCVDetectCXXCompiler.cmake has the following lines:

if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
  set(CMAKE_COMPILER_IS_GNUCXX 1)
  set(CMAKE_COMPILER_IS_CLANGCXX 1)
endif()
if(CMAKE_C_COMPILER_ID STREQUAL "Clang")
  set(CMAKE_COMPILER_IS_GNUCC 1)
  set(CMAKE_COMPILER_IS_CLANGCC 1)
endif()

So if it's Clang it sets it also to be gcc!

[adishavit]

See git "blame" here.

cc: @taka-no-me

[adishavit]

Apparently resolved by #10073.