mruby-io: Fixing compilation issue under the legacy MinGW environment

Adding MRB_MINGW32_LEGACY in common.h in order to identify the legacy MinGW environment (i.e. NOT to be confused with MinGW-w64).
For more info about MinGW defined macros, see: https://sourceforge.net/p/predef/wiki/Compilers/

include/mruby/common.h

@@ -82,6 +82,9 @@ MRB_BEGIN_DECL
 # elif defined(__MINGW32_MAJOR_VERSION)
 #  define MRB_MINGW32_VERSION  (__MINGW32_MAJOR_VERSION * 1000 + __MINGW32_MINOR_VERSION)
 # endif
+# if defined(__MINGW32__) && !defined(__MINGW64__)
+#   define MRB_MINGW32_LEGACY
+# endif
 #endif
 
 MRB_END_DECL

mrbgems/mruby-io/src/file.c

@@ -66,7 +66,7 @@
 #define LOCK_UN 8
 #endif
 
-#ifndef _WIN32
+#if !defined(_WIN32) || defined(MRB_MINGW32_LEGACY)
 typedef struct stat         mrb_stat;
 # define mrb_stat(path, sb) stat(path, sb)
 # define mrb_fstat(fd, sb)  fstat(fd, sb)

mrbgems/mruby-io/src/io.c

@@ -33,9 +33,11 @@
   typedef long fsuseconds_t;
   typedef int fmode_t;
   typedef int mrb_io_read_write_size;
-  #if !defined(_SSIZE_T_) && !defined(_SSIZE_T_DEFINED) && \
-      !defined(__have_typedef_ssize_t)
-  typedef SSIZE_T ssize_t;
+  #ifndef MRB_MINGW32_LEGACY
+    #if !defined(_SSIZE_T_) && !defined(_SSIZE_T_DEFINED) && \
+        !defined(__have_typedef_ssize_t)
+    typedef SSIZE_T ssize_t;
+    #endif
   #endif
 
   #ifndef O_TMPFILE