httpc: fix compilation with libcurl >= 7.62.0

Starting from libcurl 7.62.0, CURL_SSL_CACERT is defined as a macro
alias to CURLE_PEER_FAILED_VERIFICATION, see

  https://github.com/curl/curl/commit/3f3b26d6feb0667714902e836af608094235fca2

This breaks compilation:

  httpc.c:337:7: error: duplicate case value 'CURLE_PEER_FAILED_VERIFICATION'
          case CURLE_PEER_FAILED_VERIFICATION:
               ^
  httpc.c:336:7: note: previous case defined here
          case CURLE_SSL_CACERT:
               ^
  curl.h:589:26: note: expanded from macro 'CURLE_SSL_CACERT'
  #define CURLE_SSL_CACERT CURLE_PEER_FAILED_VERIFICATION
                           ^

Fix this by using CURLE_SSL_CACERT only if libcurl version is less
than 7.62.0.

Note, we can't use CURL_AT_LEAST_VERSION to check libcurl version,
because it isn't available in libcurl shipped with CentOS 6.