Disable conditional compilation of 'pause' feature

PycURL already requires cURL 7.19.0. Checking for
cURL 7.18.0 is therefore not required to enable
curl_easy_pause() function and related options.
Define helper HAVE_CURLOPT_DNS_SERVERS

Instead of checking against LIBCURL_VERSION_NUM
every time define the helper
HAVE_CURLOPT_DNS_SERVERS.
Merge pull request #65 from romuloceccon/feature_detection_cleanup

Feature detection cleanup