fixed boo#1020046: xsp build should fail but not.
in mono 4.6.x the search path has changed from
/usr/lib/mono/2.0 to /usr/lib/mono/2.0-api, so
every build relying on the old search path will
and should fail. But for some unkown reasons I
think xsp didn't rebuild after mono-core's version
bump, leaving an unpredictable build success and
a package that may be not usable at all. This
backport will fix the build time problem and
generate a correct package.