Problem

On MSVC, M_PI is not declared by <cmath> unless _USE_MATH_DEFINES is defined before the header is included, so the build fails with identifier "M_PI" is undefined (#6). The constant is used in src/fft.cpp, src/mel_gpu.cpp, and the test sources.

Fix

• In src/fft.cpp and src/mel_gpu.cpp, define _USE_MATH_DEFINES before <cmath> and add an #ifndef M_PI fallback, so the library sources compile regardless of toolchain or include ordering.
• In CMakeLists.txt, add _USE_MATH_DEFINES for MSVC as a PUBLIC compile definition on the parakeet target. PUBLIC (rather than PRIVATE) propagates it to consumers that link the library, so the test executables built with -DPARAKEET_BUILD_TESTS=ON (test_fft, test_audio_io, test_mel_gpu, which also use M_PI) get the define as well.

Non-MSVC builds are unaffected: the generator expression only emits the define for CXX_COMPILER_ID:MSVC, and the #ifndef guards are no-ops where M_PI is already provided.

Testing

• Constant/define-only change; no runtime logic is modified.
• Verified the generator expression is scoped to MSVC and that the PUBLIC definition reaches the test targets through target_link_libraries(<test> PRIVATE parakeet).

Closes #6

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