Use fseeko64/ftello64 for mingw32.

It turns out i686-pc-mingw32 does define these functions, and they
 are always available (unlike _fseeki64/_ftelli64).
This means we can build and link without requiring
 i686-w64-mingw32.
The resulting binary still doesn't run in wine for me, but that may
 be a personal problem.

src/stream.c

@@ -56,7 +56,18 @@ static int op_fread(void *_stream,unsigned char *_ptr,int _buf_size){
 static int op_fseek(void *_stream,opus_int64 _offset,int _whence){
 #if defined(_MSC_VER)
   return _fseeki64((FILE *)_stream,_offset,_whence);
+#elif defined(__MINGW32__)
+  /*i686-pc-mingw32 does not have fseeko() and requires
+     __MSVCRT_VERSION__>=0x800 for _fseeki64(), which screws up linking with
+     other libraries (that don't use MSVCRT80 from MSVC 2005 by default).
+    i686-w64-mingw32 does have fseeko() and respects _FILE_OFFSET_BITS, but I
+     don't know how to detect that at compile time.
+    We don't need to use fopen64(), as this just dispatches to fopen() in
+     mingw32.*/
+  return fseeko64((FILE *)_stream,(off64_t)_offset,_whence);
 #else
+  /*This function actually conforms to the SUSv2 and POSIX.1-2001, so we prefer
+     it except in the two special-cases above.*/
   return fseeko((FILE *)_stream,(off_t)_offset,_whence);
 #endif
 }
@@ -64,7 +75,18 @@ static int op_fseek(void *_stream,opus_int64 _offset,int _whence){
 static opus_int64 op_ftell(void *_stream){
 #if defined(_MSC_VER)
   return _ftelli64((FILE *)_stream);
+#elif defined(__MINGW32__)
+  /*i686-pc-mingw32 does not have ftello() and requires
+     __MSVCRT_VERSION__>=0x800 for _ftelli64(), which screws up linking with
+     other libraries (that don't use MSVCRT80 from MSVC 2005 by default).
+    i686-w64-mingw32 does have ftello() and respects _FILE_OFFSET_BITS, but I
+     don't know how to detect that at compile time.
+    We don't need to use fopen64(), as this just dispatches to fopen() in
+     mingw32.*/
+  return ftello64((FILE *)_stream);
 #else
+  /*This function actually conforms to the SUSv2 and POSIX.1-2001, so we prefer
+     it except in the two special-cases above.*/
   return ftello((FILE *)_stream);
 #endif
 }