Fix FTBFS with std=c++0x (C++11)

It will fix the following error:

/usr/include/absl/base/policy_checks.h:79:2: error: #error "C++ versions less than C++14 are not supported."

It seems that -std=c++0x (C++11) is explicitly given in debian/rules, so newer absl library reject it.

ref. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1068202

Signed-off-by: Kentaro Hayashi kenhys@gmail.com