dbus-cpp: fix cross build for tools when expat is missing

dbus-c++ tools use CXX_FOR_BUILD to build in cross scenarios, however
they don't use CXXFLAGS_FOR_BUILD nor LDFLAGS_FOR_BUILD thus breaking
when there aren't any expat devel files in the default PATHs (build
host). Fixes:
http://autobuild.buildroot.net/results/44f/44fc2cab2b60aa82460bb46b1035ddda115af750/



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