package/dbus-cpp: fix cross-compilation

package/dbus-cpp: fix cross-compilation

dbus-cpp has an ugly hack to not cross-build the tools when it detects
cross-compilation.

However, we already have a host variant that builds those tools (and we
anyway don't seem to need them to begin with).

Drop our patch that propagates the CXXFLAGS/LDFLAGS_FOR_BUILD. Replace
with a patch to cross-compile the tools.

Fixes:
    http://autobuild.buildroot.org/results/110/1100539caae6ef62c61a3b96bc54f7c0a215cc62/
    and many, many more.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>