package/doxygen: needs host gcc >= 4.9

host-doxygen use std::make_unique which is a C++14 feature and so not
available with host gcc 4.8 so add a Config.in.host for doxygen and add
host gcc 4.9 dependency to host-doxygen and sigrok C++ option

Fixes:
 - http://autobuild.buildroot.org/results/3ac78c5d4728287bafdfeb3a54f50eb193934b63



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