It would be nice to also change signbit definition on systems that
    doesn't support it, so it at least works for normal numbers.

The signbit() function is actually part of C99, not my implementation,
so I think it should be supported on most systems? Though I am not quite
sure...

It's not available when compiled with Visual Studio. There's following code in math.c:
    #ifndef signbit
    #define signbit(x) 0
    #endif
I think it would be better if it's at least changed to something like:
#define signbit(x) ((x)<0 ? 1 : 0)
It will not work correctly for -NAN or -0 but it will not break the cases where old implementation was working.

Sorry I didn't notice my program in VS was linked to the C++ lib for this signbit() function.

"#define signbit(x) ((x)<0 ? 1 : 0)"  Looks good to me, I will submit a patch. Thanks for the advice :)