matchbox-keyboard: fix dependencies

matchbox-keyboard: fix dependencies

Correct a number of issues with matchbox-keyboard:
 - Missing dependencies on host-pkgconf and expat
 - Missing kconfig select on xlib_libXft
 - pango/cairo confusion, ensure cairo is built before if enabled
 - _CONF_OPT variable typo

Fixes:
http://autobuild.buildroot.net/results/473/47380d57e507ec395a1132dfd7b32d35459c1c7a//
http://autobuild.buildroot.net/results/d9a/d9aa2b6ddce920ea45fbd7093addfc0ee6b94467//
http://autobuild.buildroot.net/results/aad/aad393a81812df28cbc296f556de2acb4e6d114b//
http://autobuild.buildroot.net/results/124/124b28336ed61822fa6d5d2591b4e70d83b8a213//
and 17 other build errors, dating back to 2013-07-03

Reported-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>