cmake: keep strict-prototypes on C flags

[Old-Ding]

Summary

• Keep -Wstrict-prototypes on C compiler flags only.

Details

The strict_prototypes option enables GCC/Clang's -Wstrict-prototypes, which is a C-only diagnostic. The current CMake logic appends it to both CMAKE_C_FLAGS and CMAKE_CXX_FLAGS, so enabling the option also passes a C-only warning flag to C++ targets.

This leaves the C behavior unchanged and avoids adding the C-only option to C++ compiler invocations.

Testing

• git diff --check HEAD~1..HEAD
• Reviewed the strict_prototypes CMake option path and C++ test/sample targets that consume CMAKE_CXX_FLAGS.

Not run: local CMake configure/build, because this Windows environment does not have cmake or a C/C++ compiler installed.