MingW uses gcc. Hence the suffixes for 64-bit are 'LL' and 'ULL'.

include/curl/curlbuild.h.dist

@@ -31,7 +31,7 @@
  * NOTE 1:
  * -------
  *
- * See file include/curl/curlbuild.h.in, run configure, and forget 
+ * See file include/curl/curlbuild.h.in, run configure, and forget
  * that this file exists it is only used for non-configure systems.
  * But you can keep reading if you want ;-)
  *
@@ -230,7 +230,7 @@
 #    define CURL_SUFFIX_CURL_OFF_TU ULL
 #  endif
 
-#elif defined(__LCC__) 
+#elif defined(__LCC__)
 #  define CURL_OFF_T              long
 #  define CURL_FMT_OFF_T          "ld"
 #  define CURL_FMT_OFF_TU         "lu"
@@ -291,8 +291,8 @@
 #  define CURL_FMT_OFF_TU         "I64u"
 #  define CURL_FORMAT_OFF_T       "%I64d"
 #  define CURL_SIZEOF_CURL_OFF_T  8
-#  define CURL_SUFFIX_CURL_OFF_T  i64
-#  define CURL_SUFFIX_CURL_OFF_TU ui64
+#  define CURL_SUFFIX_CURL_OFF_T  LL
+#  define CURL_SUFFIX_CURL_OFF_TU ULL
 
 #elif defined(_MSC_VER)
 #  if (_MSC_VER >= 900)