[flang] [runtime] Fix build warnings if built with mingw

[mstorsjo]

Summary

Check whether F_OK et al are defined before redefining them; mingw
headers do define them, so check before providing the windows fallback
defines.

Also check _WIN32 instead of WIN32; this is how it's consistently
done in the rest of llvm. (The former is a compiler builtin define,
while the latter isn't, but it's commonly set by e.g. build systems.)

[mmuetzel]

I'm not sure how much my review is worth. But this change looks good to me. 👍

[awarzynski]

This is not my area of expertise, but it looks fairly straightforward and makes sense to me. Thanks, LGTM!

flang/runtime/file.cpp

if (fd_ <= 2) {
       }
     }
     fd_ = -1;
   }
 }
 
 bool IsATerminal(int fd) { return ::isatty(fd); }
 
-#ifdef WIN32
+#if defined(_WIN32) && !defined(F_OK)
 // Access flags are normally defined in unistd.h, which unavailable under
 // Windows. Instead, define the flags as documented at
 // https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/access-waccess
+// On Mingw, io.h does define these same constants - so check whether they
+// already are defined before defining these.
 #define F_OK 00
 #define W_OK 02
 #define R_OK 04
 #endif
 
 bool IsExtant(const char *path) { return ::access(path, F_OK) == 0; }
 bool MayRead(const char *path) { return ::access(path, R_OK) == 0; }
 bool MayWrite(const char *path) { return ::access(path, W_OK) == 0; }