Note that for some reason there is this warning (that also exists with
autotools, added since curl-7_15_1-94-ga718cb05f):
docs/libcurl/curl_multi_socket_all.3:1: can't open `man3/curl_multi_socket.3': No such file or directory
Additionally, adjust the roffit --mandir option to support creating
links when doing out-of-tree builds.
Ref: https://github.com/curl/curl/pull/1288
