Merge branch 'tc/curl-with-backports'

Updates to the HTTP layer we made recently unconditionally used
features of libCurl without checking the existence of them, causing
compilation errors, which has been fixed.  Also migrate the code to
check feature macros, not version numbers, to cope better with
libCurl that vendor ships with backported features.

* tc/curl-with-backports:
  http: use a feature check to enable GSSAPI delegation control
  http: fix handling of missing CURLPROTO_*

diff --cc http.c
Simple merge