Fix: compilation Issue on ARM64 (msys2 clangarm)

[thiru31]

Added a definition for M_PI in the code to resolve a compilation error encountered when building OpenCV on the MSYS2 environment. The M_PI constant was not defined, causing the compilation to fail.

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• [x] I agree to contribute to the project under Apache 2 License.
• [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
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[Kumataro]

Hello, my sample CMakeList.txt has not been tested, so if there are some build error, I'm sorry.

---

M_PI is not a part of C++(and C) Standard. See https://stackoverflow.com/questions/51344013/m-pi-imprecision

(I know this is only armchair theory, not reality.) For example, if someone uses compiler that has -DM_PI=42, this fix will not works well.

Instead of definiton M_PI in user source code, we can use M_PI, with "_USE_MATH_DEFINES` definition.

https://github.com/opencv/opencv/blob/4.x/3rdparty/carotene/CMakeLists.txt

      endif()
  endif()

+ if(MINGW) # TODO: What condition is better ?
+    target_compile_definitions(carotene_objs PRIVATE "-D_USE_MATH_DEFINES=1")
+ endif()

  # we add dummy file to fix XCode build
  add_library(carotene STATIC ${OPENCV_3RDPARTY_EXCLUDE_FROM_ALL} "$<TARGET_OBJECTS:carotene_objs>" 
  "${CAROTENE_SOURCE_DIR}/dummy.cpp")

Or Add #define _USE_MATH_DEFINES before #include <cmath> in phase.cpp

[thiru31]

@Kumataro I have updated the PR to use _USE_MATH_DEFINES as suggested, by adding the following to CMakeLists.txt:

if(MINGW)
    target_compile_definitions(carotene_objs PRIVATE "-D_USE_MATH_DEFINES=1")
endif()

I have compiled and tested this change on my local ARM machine, and it resolves the compilation issue successfully.

Please review the changes and let me know if any further adjustments are needed. Thank you!

[Kumataro]

I think the new patch is preferable as it limits the impact to the MINGW environment only. Thank you very much !