Allow building without gnu extensions

[fineless71]

My feature request is to support building without gnu extensions. Currently, I have to build with -std=gnu89 instead of -std=c89 (or do the work around below). Here's the error I get with c89:

src/../lib/minifs.h:2175:28: error: use of undeclared identifier 'S_IFDIR'
 2175 |     return (info.st_mode & S_IFDIR) != 0;
      |                            ^
src/../lib/minifs.h:2188:28: error: use of undeclared identifier 'S_IFDIR'
 2188 |     return (info.st_mode & S_IFDIR) == 0;
      |                            ^            

I currently do this work around to get around this:

#define MINIFS_IMPLEMENTATION
/* c89 compat (normally requires gnu89) */
#include <sys/stat.h>
#ifndef S_IFDIR
#define S_IFDIR __S_IFDIR
#endif
#include "../lib/minifs.h"
#undef MINIFS_IMPLEMENTATION

I would make a pull request, but I do not think __S_IFDIR is a portable name, and I do not know the correct way to fix this.

Edit: I believed I found out the correct way and made a pull request: #5