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>

package/libgsasl/libgsasl.mk

@@ -9,6 +9,14 @@ LIBGSASL_SITE = $(BR2_GNU_MIRROR)/gsasl
 LIBGSASL_LICENSE = LGPLv2.1+ (library), GPLv3+ (programs)
 LIBGSASL_LICENSE_FILES = README COPYING.LIB COPYING
 LIBGSASL_INSTALL_STAGING = YES
-LIBGSASL_DEPENDENCIES = $(if $(BR2_PACKAGE_LIBIDN),libidn)
+# It doesn't seem to build with our libgcrypt so better be safe
+LIBGSASL_CONF_OPTS = --without-libgcrypt
+
+ifeq ($(BR2_PACKAGE_LIBIDN),y)
+LIBGSASL_CONF_OPTS += --with-libidn-prefix=$(STAGING_DIR)/usr
+LIBGSASL_DEPENDENCIES += libidn
+else
+LIBGSASL_CONF_OPTS += --without-stringprep
+endif
 
 $(eval $(autotools-package))