add the GLOBAL_ACK_EINTR constant to the list of exported symbols

... if built against a new enough version of libcurl

Bug: https://bugzilla.redhat.com/920589

Signed-off-by: Kamil Dudka <kdudka@redhat.com>
tests/global_init_ack_eintr.py: test GLOBAL_ACK_EINTR

... if we have a new enough version of libcurl