CMAKE_SIZEOF_VOID_P incorrectly detected on mingw cross compiler

[alextwothousand]

Context:
OS: Arch Linux x86_64
Compiler: x86_64-mingw-w64-gcc

Issue:
CMAKE_SIZEOF_VOID_P appears to be detected incorrectly on the mingw cross compiler. It is including hde32.c, rather than hde64, and I have confirmed this by displaying a CMake fatal error. I would implement a fix through a PR, but I'm unsure of what the goal for this is - to check whether you're running a 64 bit architecture, or a 32 bit architecture? Thank you.

[alextwothousand]

Check alextwothousand@a030771 on my fork, it works fine now with my FORCE_USE_HDE64 toggle. Obviously not a perm fix, but works fine for me now.