This is because sse2neon.h might be used to emulate SSE intrinsics
on ARM64 architecture, and it uses some preprocessor which is not
available for C language when using MSVC.
The old-style math file math_matrix.c uses this header, so needed
to become C++. Simple rename did not work since there is a new math
utility math_matrix.cc exists. Following some existing convention
the math_matrix.c is renamed to math_matrix_c.cc. Eventually all the
code should switch to use C++ style math, and the C style removed,
so it seems reasonable to not mix old and new style of API in the
same file.
There should be no functional changes.
Pull Request: https://projects.blender.org/blender/blender/pulls/121335
1b0012d51c