fixed build error with CentOS 5

I got following error on CentOS 5

```
curb_easy.c: In function 'ruby_curl_easy_put_data_set':
curb_easy.c:794: error: 'CURLOPT_SEEKFUNCTION' undeclared (first use in this function)
curb_easy.c:794: error: (Each undeclared identifier is reported only once
curb_easy.c:794: error: for each function it appears in.)
curb_easy.c:794: error: 'curl_seek_callback' undeclared (first use in this function)
curb_easy.c:794: error: expected ')' before 'seek_data_handler'
curb_easy.c:796: error: 'CURLOPT_SEEKDATA' undeclared (first use in this function)
(snip)
make: *** [curb_easy.o] Error 1
```

this issue caused by [this commit](https://github.com/taf2/curb/commit/b092bcbc2e0c8382992d3c1ce54220235fe5baac)

I ignored some function with undeclared options on old environment like centos5.