libgsasl: fix auto dependencies

IDN can be picked up from the distro installation so use a prefix when
it's available or otherwise just disable it, fixes:
http://autobuild.buildroot.org/results/5d9/5d9baf528c47c5167ec8d2a6941cb06b4ca761ca/



Also disable libgcrypt support which wasn't accounted for and doesn't
seem to work either to possibly avoid the same problem.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>