curlftpfs: specify curl-config path

curlftpfs may pick up the distribution curl-config rather than the one
in the staging area thus failing to build.
Fix it by hardcoding the curl-config path.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>