Fix compilation on Linux

[JayFoxRox]

I did not try to run it yet and I also can't test Windows support right now. Please test and report issues before merging.

• Do not use SYSTEM for our own include files
• Replace existing dirent.h for Windows with the new one, remove duplicate
• Include <sys/stat.h> for mkdir
• Use do_mkdir wrapper
• Don't cast argument for scandir (_cdecl not supported on Posix). If this breaks windows, replace dirent prototype with int scandir(const char *dirp, struct dirent ***namelist, int (*filter)(const struct dirent *), int (*compar)(const struct dirent **, const struct dirent **)); (from Linux manpage)
• Include math.h for floorf
• Fix path seperators from \ to /

[JayFoxRox]

Pathname seperators are bad. My filenames are out_modelblock\model_20.bin (that's a file called like that, not a file called model_20.bin in folder out_modelblock)