xapp_xclock: fix build with pkgconf 1.5.3

xapp_xclock: fix build with pkgconf 1.5.3

Since the bump to 1.5.3, pkgconf prepends the sysroot to all absolute
paths found in the .pc file. This is correct when the paths refer to
something in STAGING_DIR (e.g. libdir, includedir), but not when it
refers to something used for the target.

xapp_xclock uses the appdefaultdir variable from xt.pc to decide where
to install things. Since DESTDIR is prepended to the install
destination, this will end up in the wrong location.

Until a better solution is found in pkgconf, pass the appdefaultdir to
use explicitly instead of relying on xt.pc.

Fixes:
 - http://autobuild.buildroot.org/results/ea028467a981f419c395158c55aa9a6d16e3f2c1

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>