Fix curl_off_t definition for builds done using Sun compilers and a

non-configured libcurl. In this case curl_off_t data type was gated
to the off_t data type which depends on the _FILE_OFFSET_BITS. This
configuration is exactly the unwanted configuration for our curl_off_t
data type which must not depend on such setting. This breaks ABI for
libcurl libraries built with Sun compilers which were built without
having run the configure script with _FILE_OFFSET_BITS different than
64 and using the ILP32 data model.